<!--
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.301.4.11 2010/03/17 18:04:09 mha Exp $
-->

<chapter id="runtime">
 <title>Server Run-time Environment</title>

 <para>
  This chapter discusses how to set up and run the database server
  and its interactions with the operating system.
 </para>

 <sect1 id="postgres-user">
  <title>The <productname>PostgreSQL</productname> User Account</title>

  <indexterm>
   <primary>postgres user</primary>
  </indexterm>

  <para>
   As with any other server daemon that is accessible to the outside world,
   it is advisable to run <productname>PostgreSQL</productname> under a
   separate user account. This user account should only own the data
   that is managed by the server, and should not be shared with other
   daemons. (For example, using the user <literal>nobody</literal> is a bad
   idea.) It is not advisable to install executables owned by this 
   user because compromised systems could then modify their own 
   binaries.
  </para>

  <para>
   To add a Unix user account to your system, look for a command
   <command>useradd</command> or <command>adduser</command>. The user
   name <systemitem>postgres</systemitem> is often used, and is assumed
   throughout this book, but you can use another name if you like.
  </para>
 </sect1>

 <sect1 id="creating-cluster">
  <title>Creating a Database Cluster</title>

  <indexterm>
   <primary>database cluster</primary>
  </indexterm>

  <indexterm>
   <primary>data area</primary>
   <see>database cluster</see>
  </indexterm>

  <para>
   Before you can do anything, you must initialize a database storage
   area on disk. We call this a <firstterm>database cluster</firstterm>.
   (<acronym>SQL</acronym> uses the term catalog cluster.) A
   database cluster is a collection of databases that is managed by a
   single instance of a running database server. After initialization, a
   database cluster will contain a database named
   <literal>template1</literal>. As the name suggests, this will be used
   as a template for subsequently created databases; it should not be
   used for actual work.  (See <xref linkend="managing-databases"> for
   information about creating new databases within a cluster.)
  </para>

  <para>
   In file system terms, a database cluster will be a single directory
   under which all data will be stored. We call this the <firstterm>data
   directory</firstterm> or <firstterm>data area</firstterm>. It is
   completely up to you where you choose to store your data.  There is no
   default, although locations such as
   <filename>/usr/local/pgsql/data</filename> or
   <filename>/var/lib/pgsql/data</filename> are popular. To initialize a
   database cluster, use the command <xref
   linkend="app-initdb">,<indexterm><primary>initdb</></> which is
   installed with <productname>PostgreSQL</productname>. The desired
   file system location of your database cluster is indicated by the
   <option>-D</option> option, for example
<screen>
<prompt>$</> <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
   Note that you must execute this command while logged into the
   <productname>PostgreSQL</productname> user account, which is
   described in the previous section.
  </para>

  <tip>
   <para>
    As an alternative to the <option>-D</option> option, you can set
    the environment variable <envar>PGDATA</envar>.
    <indexterm><primary><envar>PGDATA</envar></primary></indexterm>
   </para>
  </tip>

  <para>
   <command>initdb</command> will attempt to create the directory you
   specify if it does not already exist. It is likely that it will not
   have the permission to do so (if you followed our advice and created
   an unprivileged account). In that case you should create the
   directory yourself (as root) and change the owner to be the
   <productname>PostgreSQL</productname> user. Here is how this might
   be done:
<screen>
root# <userinput>mkdir /usr/local/pgsql/data</userinput>
root# <userinput>chown postgres /usr/local/pgsql/data</userinput>
root# <userinput>su postgres</userinput>
postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
  </para>

  <para>
   <command>initdb</command> will refuse to run if the data directory
   looks like it has already been initialized.</para>

  <para>
   Because the data directory contains all the data stored in the
   database, it is essential that it be secured from unauthorized
   access. <command>initdb</command> therefore revokes access
   permissions from everyone but the
   <productname>PostgreSQL</productname> user.
  </para>

  <para>
   However, while the directory contents are secure, the default
   client authentication setup allows any local user to connect to the
   database and even become the database superuser. If you do not
   trust other local users, we recommend you use one of
   <command>initdb</command>'s <option>-W</option>, <option>--pwprompt</option>
   or <option>--pwfile</option> options to assign a password to the
   database superuser.<indexterm><primary>password</><secondary>of the
   superuser</></indexterm>  Also, specify <option>-A md5</> or
   <option>-A password</> so that the default <literal>trust</> authentication
   mode is not used; or modify the generated <filename>pg_hba.conf</filename>
   file after running <command>initdb</command>,
   <emphasis>before</> you start the server for the first time. (Other
   reasonable approaches include using <literal>ident</literal> authentication
   or file system permissions to restrict connections. See <xref
   linkend="client-authentication"> for more information.)
  </para>

  <para>
   <command>initdb</command> also initializes the default
   locale<indexterm><primary>locale</></> for the database cluster.
   Normally, it will just take the locale settings in the environment
   and apply them to the initialized database.  It is possible to
   specify a different locale for the database; more information about
   that can be found in <xref linkend="locale">.  The sort order used
   within a particular database cluster is set by
   <command>initdb</command> and cannot be changed later, short of
   dumping all data, rerunning <command>initdb</command>, and reloading
   the data. There is also a performance impact for using locales
   other than <literal>C</> or <literal>POSIX</>. Therefore, it is
   important to make this choice correctly the first time.
  </para>

  <para>
   <command>initdb</command> also sets the default character set encoding
   for the database cluster.  Normally this should be chosen to match the
   locale setting.  For details see <xref linkend="multibyte">.
  </para>
 </sect1>

 <sect1 id="postmaster-start">
  <title>Starting the Database Server</title>

  <para>
   Before anyone can access the database, you must start the database
   server. The database server program is called
   <command>postmaster</command>.<indexterm><primary>postmaster</></>
   The <command>postmaster</command> must know where to
   find the data it is supposed to use. This is done with the
   <option>-D</option> option. Thus, the simplest way to start the
   server is:
<screen>
$ <userinput>postmaster -D /usr/local/pgsql/data</userinput>
</screen>
   which will leave the server running in the foreground. This must be
   done while logged into the <productname>PostgreSQL</productname> user
   account. Without <option>-D</option>, the server will try to use
   the data directory named by the environment variable <envar>PGDATA</envar>.
   If that variable is not provided either, it will fail.
  </para>

  <para>
   Normally it is better to start the <command>postmaster</command> in the
   background.  For this, use the usual shell syntax:
<screen>
$ <userinput>postmaster -D /usr/local/pgsql/data &gt;logfile 2&gt;&amp;1 &amp;</userinput>
</screen>
   It is important to store the server's <systemitem>stdout</> and
   <systemitem>stderr</> output somewhere, as shown above. It will help
   for auditing purposes and to diagnose problems. (See <xref
   linkend="logfile-maintenance"> for a more thorough discussion of log
   file handling.)
  </para>

  <para>
   The <command>postmaster</command> also takes a number of other
   command line options. For more information, see the
   <xref linkend="app-postmaster"> reference page
   and <xref linkend="runtime-config"> below.
  </para>

  <para>
   This shell syntax can get tedious quickly.  Therefore the wrapper
   program
   <xref linkend="app-pg-ctl"><indexterm><primary>pg_ctl</primary></indexterm>
   is provided to simplify some tasks.  For example:
<programlisting>
pg_ctl start -l logfile
</programlisting>
   will start the server in the background and put the output into the
   named log file. The <option>-D</option> option has the same meaning
   here as in the <command>postmaster</command>. <command>pg_ctl</command>
   is also capable of stopping the server.
  </para>

  <para>
   Normally, you will want to start the database server when the
   computer boots.<indexterm><primary>booting</><secondary>starting
   the server during</></> Autostart scripts are operating-system-specific.
   There are a few distributed with
   <productname>PostgreSQL</productname> in the
   <filename>contrib/start-scripts</> directory. Installing one will require
   root privileges.
  </para>

  <para>
   Different systems have different conventions for starting up daemons
   at boot time. Many systems have a file
   <filename>/etc/rc.local</filename> or
   <filename>/etc/rc.d/rc.local</filename>. Others use
   <filename>rc.d</> directories. Whatever you do, the server must be
   run by the <productname>PostgreSQL</productname> user account
   <emphasis>and not by root</emphasis> or any other user. Therefore you
   probably should form your commands using <literal>su -c '...'
   postgres</literal>.  For example:
<programlisting>
su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres
</programlisting>
  </para>

  <para>
   Here are a few more operating-system-specific suggestions. (In each
   case be sure to use the proper installation directory and user
   name where we show generic values.)

   <itemizedlist>
    <listitem>
     <para>
      For <productname>FreeBSD</productname>, look at the file
      <filename>contrib/start-scripts/freebsd</filename> in the
      <productname>PostgreSQL</productname> source distribution.
      <indexterm><primary>FreeBSD</><secondary>start script</secondary></>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>OpenBSD</productname>, add the following lines
      to the file <filename>/etc/rc.local</filename>:
      <indexterm><primary>OpenBSD</><secondary>start script</secondary></>
<programlisting>
if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postmaster ]; then
    su - -c '/usr/local/pgsql/bin/pg_ctl start -l /var/postgresql/log -s' postgres
    echo -n ' postgresql'
fi
</programlisting>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>Linux</productname> systems either add
      <indexterm><primary>Linux</><secondary>start script</secondary></>
<programlisting>
/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data
</programlisting>
      to <filename>/etc/rc.d/rc.local</filename> or look at the file
      <filename>contrib/start-scripts/linux</filename> in the
      <productname>PostgreSQL</productname> source distribution.
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>NetBSD</productname>, either use the
      <productname>FreeBSD</productname> or
      <productname>Linux</productname> start scripts, depending on
      preference. <indexterm><primary>NetBSD</><secondary>start script</secondary></>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>Solaris</productname>, create a file called
      <filename>/etc/init.d/postgresql</filename> that contains
      the following line:
      <indexterm><primary>Solaris</><secondary>start script</secondary></>
<programlisting>
su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data"
</programlisting>
      Then, create a symbolic link to it in <filename>/etc/rc3.d</> as
      <filename>S99postgresql</>.
     </para>
    </listitem>
   </itemizedlist>

  </para>

   <para>
    While the <command>postmaster</command> is running, its
    <acronym>PID</acronym> is stored in the file
    <filename>postmaster.pid</filename> in the data directory. This is
    used to prevent multiple <command>postmaster</command> processes
    running in the same data directory and can also be used for
    shutting down the <command>postmaster</command> process.
   </para>

   <sect2 id="postmaster-start-failures">
    <title>Server Start-up Failures</title>

    <para>
     There are several common reasons the server might fail to
     start. Check the server's log file, or start it by hand (without
     redirecting standard output or standard error) and see what error
     messages appear. Below we explain some of the most common error
     messages in more detail.
    </para>

    <para>
<screen>
LOG:  could not bind IPv4 socket: Address already in use
HINT:  Is another postmaster already running on port 5432? If not, wait a few seconds and retry.
FATAL:  could not create TCP/IP listen socket
</screen>
     This usually means just what it suggests: you tried to start
     another <command>postmaster</command> on the same port where one is already running.
     However, if the kernel error message is not <computeroutput>Address
     already in use</computeroutput> or some variant of that, there may
     be a different problem. For example, trying to start a <command>postmaster</command>
     on a reserved port number may draw something like:
<screen>
$ <userinput>postmaster -p 666</userinput>
LOG:  could not bind IPv4 socket: Permission denied
HINT:  Is another postmaster already running on port 666? If not, wait a few seconds and retry.
FATAL:  could not create TCP/IP listen socket
</screen>
    </para>

    <para>
     A message like
<screen>
FATAL:  could not create shared memory segment: Invalid argument
DETAIL:  Failed system call was shmget(key=5440001, size=4011376640, 03600).
</screen>
     probably means your kernel's limit on the size of shared memory is
     smaller than the work area <productname>PostgreSQL</productname>
     is trying to create (4011376640 bytes in this example). Or it could
     mean that you do not have System-V-style shared memory support
     configured into your kernel at all. As a temporary workaround, you
     can try starting the server with a smaller-than-normal number
     of buffers (<option>-B</option> switch). You will eventually want
     to reconfigure your kernel to increase the allowed shared memory
     size. You may also see this message when trying to start multiple
     servers on the same machine, if their total space requested
     exceeds the kernel limit.
    </para>

    <para>
     An error like
<screen>
FATAL:  could not create semaphores: No space left on device
DETAIL:  Failed system call was semget(5440126, 17, 03600).
</screen>
     does <emphasis>not</emphasis> mean you've run out of disk
     space. It means your kernel's limit on the number of <systemitem
     class="osname">System V</> semaphores is smaller than the number
     <productname>PostgreSQL</productname> wants to create. As above,
     you may be able to work around the problem by starting the
     server with a reduced number of allowed connections
     (<option>-N</option> switch), but you'll eventually want to
     increase the kernel limit.
    </para>

    <para>
     If you get an <quote>illegal system call</> error, it is likely that
     shared memory or semaphores are not supported in your kernel at
     all. In that case your only option is to reconfigure the kernel to
     enable these features.
    </para>

    <para>
     Details about configuring <systemitem class="osname">System V</>
     <acronym>IPC</> facilities are given in <xref linkend="sysvipc">.
    </para>
   </sect2>

   <sect2 id="client-connection-problems">
    <title>Client Connection Problems</title>

    <para>
     Although the error conditions possible on the client side are quite
     varied and application-dependent, a few of them might be directly
     related to how the server was started up. Conditions other than
     those shown below should be documented with the respective client
     application.
    </para>

    <para>
<screen>
psql: could not connect to server: Connection refused
        Is the server running on host "server.joe.com" and accepting
        TCP/IP connections on port 5432?
</screen>
     This is the generic <quote>I couldn't find a server to talk
     to</quote> failure. It looks like the above when TCP/IP
     communication is attempted. A common mistake is to forget to
     configure the server to allow TCP/IP connections.
    </para>

    <para>
     Alternatively, you'll get this when attempting Unix-domain socket
     communication to a local server:
<screen>
psql: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
</screen>
    </para>

    <para>
     The last line is useful in verifying that the client is trying to
     connect to the right place. If there is in fact no server
     running there, the kernel error message will typically be either
     <computeroutput>Connection refused</computeroutput> or
     <computeroutput>No such file or directory</computeroutput>, as
     illustrated. (It is important to realize that
     <computeroutput>Connection refused</computeroutput> in this context
     does <emphasis>not</emphasis> mean that the server got your
     connection request and rejected it. That case will produce a
     different message, as shown in <xref
     linkend="client-authentication-problems">.) Other error messages
     such as <computeroutput>Connection timed out</computeroutput> may
     indicate more fundamental problems, like lack of network
     connectivity.
    </para>
   </sect2>
  </sect1>

  <sect1 id="runtime-config">
   <title>Run-time Configuration</title>

   <indexterm>
    <primary>configuration</primary>
    <secondary>of the server</secondary>
   </indexterm>

   <para>
    There are a lot of configuration parameters that affect the
    behavior of the database system. In this subsection, we describe
    how to set configuration parameters; the following subsections
    discuss each parameter in detail.
   </para>

   <para>
    All parameter names are case-insensitive. Every parameter takes a
    value of one of four types: boolean, integer, floating point,
    or string. Boolean values may be written as <literal>ON</literal>,
    <literal>OFF</literal>, <literal>TRUE</literal>,
    <literal>FALSE</literal>, <literal>YES</literal>,
    <literal>NO</literal>, <literal>1</literal>, <literal>0</literal>
    (all case-insensitive) or any unambiguous prefix of these.
   </para>

   <para>
    One way to set these parameters is to edit the file
    <filename>postgresql.conf</><indexterm><primary>postgresql.conf</></>,
    which is normally kept in the data directory. (<command>initdb</>
    installs a default copy there.) An example of what this file might look
    like is:
<programlisting>
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '$user, public'
</programlisting>
    One parameter is specified per line. The equal sign between name and
    value is optional. Whitespace is insignificant and blank lines are
    ignored. Hash marks (<literal>#</literal>) introduce comments
    anywhere.  Parameter values that are not simple identifiers or
    numbers must be single-quoted.
   </para>

   <para>
    <indexterm>
     <primary>SIGHUP</primary>
    </indexterm>
    The configuration file is reread whenever the
    <command>postmaster</command> process receives a
    <systemitem>SIGHUP</> signal (which is most easily sent by means
    of <literal>pg_ctl reload</>). The <command>postmaster</command>
    also propagates this signal to all currently running server
    processes so that existing sessions also get the new
    value. Alternatively, you can send the signal to a single server
    process directly.  Some parameters can only be set at server start;
    any changes to their entries in the configuration file will be ignored
    until the server is restarted.
   </para>

   <para>
    A second way to set these configuration parameters is to give them
    as a command line option to the <command>postmaster</command>, such as:
<programlisting>
postmaster -c log_connections=yes -c log_destination='syslog'
</programlisting>
    Command-line options override any conflicting settings in
    <filename>postgresql.conf</filename>.  Note that this means you won't
    be able to change the value on-the-fly by editing
    <filename>postgresql.conf</filename>, so while the command-line
    method may be convenient, it can cost you flexibility later.
   </para>

   <para>
    Occasionally it is useful to give a command line option to
    one particular session only. The environment variable
    <envar>PGOPTIONS</envar> can be used for this purpose on the
    client side:
<programlisting>
env PGOPTIONS='-c geqo=off' psql
</programlisting>
    (This works for any <application>libpq</>-based client application, not
    just <application>psql</application>.) Note that this won't work for
    parameters that are fixed when the server is started or that must be
    specified in <filename>postgresql.conf</filename>.
   </para>

   <para>
    Furthermore, it is possible to assign a set of option settings to
    a user or a database.  Whenever a session is started, the default
    settings for the user and database involved are loaded.  The
    commands <xref linkend="sql-alteruser" endterm="sql-alteruser-title">
    and <xref linkend="sql-alterdatabase" endterm="sql-alterdatabase-title">,
    respectively, are used to configure these settings.  Per-database
    settings override anything received from the
    <command>postmaster</command> command-line or the configuration
    file, and in turn are overridden by per-user settings; both are
    overridden by per-session options.
   </para>

   <para>
    Some parameters can be changed in individual <acronym>SQL</acronym>
    sessions with the <xref linkend="SQL-SET" endterm="SQL-SET-title">
    command, for example:
<screen>
SET ENABLE_SEQSCAN TO OFF;
</screen>
    If <command>SET</> is allowed, it overrides all other sources of
    values for the parameter. Some parameters cannot be changed via
    <command>SET</command>: for example, if they control behavior that
    cannot reasonably be changed without restarting
    <productname>PostgreSQL</productname>.  Also, some parameters can
    be modified via <command>SET</command> or <command>ALTER</> by superusers,
    but not by ordinary users.
   </para>

   <para>
    The <xref linkend="SQL-SHOW" endterm="SQL-SHOW-title">
    command allows inspection of the current values of all parameters.
   </para>

   <para>
    The virtual table <structname>pg_settings</structname>
    (described in <xref linkend="view-pg-settings">) also allows
    displaying and updating session run-time parameters.  It is equivalent
    to <command>SHOW</> and <command>SET</>, but can be more convenient
    to use because it can be joined with other tables, or selected from using
    any desired selection condition.
   </para>
    
   <sect2 id="runtime-config-file-locations">
    <title>File Locations</title>

     <para>
      In addition to the <filename>postgresql.conf</filename> file
      already mentioned, <productname>PostgreSQL</productname> uses
      two other manually-edited configuration files, which control
      client authentication (their use is discussed in <xref
      linkend="client-authentication">).
      By default, all three configuration files are stored
      in the database cluster's data directory.  The options described
      in this subsection allow the configuration files to be placed elsewhere.
      (Doing so can ease administration.  In particular it is often
      easier to ensure that the configuration files are properly backed-up
      when they are kept separate.)
     </para>

     <variablelist>
     <varlistentry id="guc-data-directory" xreflabel="data_directory">
      <term><varname>data_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>data_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the directory to use for data storage.
         This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-config-file" xreflabel="config_file">
      <term><varname>config_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>config_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the main server configuration file
         (customarily called <filename>postgresql.conf</>).
         This option can only be set on the postmaster command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-hba-file" xreflabel="hba_file">
      <term><varname>hba_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>hba_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the configuration file for host-based authentication
         (customarily called <filename>pg_hba.conf</>).
         This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ident-file" xreflabel="ident_file">
      <term><varname>ident_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>ident_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the configuration file for
         <application>ident</> authentication
         (customarily called <filename>pg_ident.conf</>).
         This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-external-pid-file" xreflabel="external_pid_file">
      <term><varname>external_pid_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>external_pid_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the name of an additional process-id (PID) file that the
        <application>postmaster</> should create for use by server
        administration programs.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     </variablelist>

     <para>
      In a default installation, none of the above options are set explicitly.
      Instead, the
      data directory is specified by the <option>-D</option> command-line
      option or the <envar>PGDATA</envar> environment variable, and the
      configuration files are all found within the data directory.
     </para>

     <para>
      If you wish to keep the configuration files elsewhere than the
      data directory, the postmaster's <option>-D</option>
      command-line option or <envar>PGDATA</envar> environment variable
      must point to the directory containing the configuration files,
      and the <varname>data_directory</> option must be set in
      <filename>postgresql.conf</filename> (or on the command line) to show
      where the data directory is actually located.  Notice that
      <varname>data_directory</> overrides <option>-D</option> and
      <envar>PGDATA</envar> for the location
      of the data directory, but not for the location of the configuration
      files.
     </para>

     <para>
      If you wish, you can specify the configuration file names and locations
      individually using the options <varname>config_file</>,
      <varname>hba_file</> and/or <varname>ident_file</>.
      <varname>config_file</> can only be specified on the 
      <command>postmaster</command> command line, but the others can be
      set within the main configuration file.  If all three options plus
      <varname>data_directory</> are explicitly set, then it is not necessary
      to specify <option>-D</option> or <envar>PGDATA</envar>.
     </para>

     <para>
      When setting any of these options, a relative path will be interpreted
      with respect to the directory in which the <command>postmaster</command>
      is started.
     </para>
   </sect2>

   <sect2 id="runtime-config-connection">
    <title>Connections and Authentication</title>

    <sect3 id="runtime-config-connection-settings">
     <title>Connection Settings</title>

     <variablelist>

     <varlistentry id="guc-listen-addresses" xreflabel="listen_addresses">
      <term><varname>listen_addresses</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>listen_addresses</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the TCP/IP address(es) on which the server is
         to listen for connections from client applications.  
         The value takes the form of a comma-separated list of host names
         and/or numeric IP addresses.  The special entry <literal>*</>
         corresponds to all available IP interfaces.
         If the list is empty, the server does not listen on any IP interface
         at all, in which case only Unix-domain sockets can be used to connect
         to it.
         The default value is <systemitem class="systemname">localhost</>,
         which allows only local <quote>loopback</> connections to be made.
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-port" xreflabel="port">
      <term><varname>port</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>port</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The TCP port the server listens on; 5432 by default.  Note that the
        same port number is used for all IP addresses the server listens on.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-connections" xreflabel="max_connections">
      <term><varname>max_connections</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Determines the maximum number of concurrent connections to the
        database server. The default is typically 100, but may be less
        if your kernel settings will not support it (as determined
        during <application>initdb</>).  This parameter can only be
        set at server start.
       </para>

       <para>
        Increasing this parameter may cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory or semaphores than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-superuser-reserved-connections"
     xreflabel="superuser_reserved_connections">
      <term><varname>superuser_reserved_connections</varname>
      (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>superuser_reserved_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Determines the number of connection <quote>slots</quote> that
        are reserved for connections by <productname>PostgreSQL</>
        superusers.  At most <xref linkend="guc-max-connections">
        connections can ever be active simultaneously.  Whenever the
        number of active concurrent connections is at least
        <varname>max_connections</> minus
        <varname>superuser_reserved_connections</varname>, new
        connections will be accepted only for superusers.
       </para>

       <para>
        The default value is 2. The value must be less than the value of
        <varname>max_connections</varname>. This parameter can only be
        set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-directory" xreflabel="unix_socket_directory">
      <term><varname>unix_socket_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the directory of the Unix-domain socket on which the
        server is to listen for
        connections from client applications.  The default is normally
        <filename>/tmp</filename>, but can be changed at build time.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
      <term><varname>unix_socket_group</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_group</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the owning group of the Unix-domain socket.  (The owning
        user of the socket is always the user that starts the
        server.)  In combination with the option
        <varname>unix_socket_permissions</varname> this can be used as
        an additional access control mechanism for Unix-domain connections.
        By default this is the empty string, which uses the default
        group for the current user.  This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
      <term><varname>unix_socket_permissions</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_permissions</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the access permissions of the Unix-domain socket.  Unix-domain
        sockets use the usual Unix file system permission set.
        The option value is expected to be a numeric mode
        specification in the form accepted by the
        <function>chmod</function> and <function>umask</function>
        system calls.  (To use the customary octal format the number
        must start with a <literal>0</literal> (zero).)
       </para>

       <para>
        The default permissions are <literal>0777</literal>, meaning
        anyone can connect. Reasonable alternatives are
        <literal>0770</literal> (only user and group, see also
        <varname>unix_socket_group</varname>) and <literal>0700</literal>
        (only user). (Note that for a Unix-domain socket, only write
        permission matters and so there is no point in setting or revoking
        read or execute permissions.)
       </para>

       <para>
        This access control mechanism is independent of the one
        described in <xref linkend="client-authentication">.
       </para>

       <para>
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-rendezvous-name" xreflabel="rendezvous_name">
      <term><varname>rendezvous_name</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>rendezvous_name</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the <productname>Rendezvous</productname> broadcast
        name.  By default, the computer name is used, specified as an
        empty string ''.  This option is ignored if the server was not
        compiled with <productname>Rendezvous</productname> support.  This
        option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-connection-security">
     <title>Security and Authentication</title>
     
     <variablelist>
     <varlistentry id="guc-authentication-timeout" xreflabel="authentication_timeout">
      <term><varname>authentication_timeout</varname> (<type>integer</type>)</term>
      <indexterm><primary>timeout</><secondary>client authentication</></indexterm>
      <indexterm><primary>client authentication</><secondary>timeout during</></indexterm>
      <indexterm>
       <primary><varname>authentication_timeout</> configuration parameter</primary>
      </indexterm>

      <listitem>
       <para>
        Maximum time to complete client authentication, in seconds. If a
        would-be client has not completed the authentication protocol in
        this much time, the server breaks the connection. This prevents
        hung clients from occupying a connection indefinitely. This
        option can only be set at server start or in the
        <filename>postgresql.conf</filename> file. The default is 60.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-ssl" xreflabel="ssl">
      <term><varname>ssl</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>ssl</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables <acronym>SSL</> connections. Please read
        <xref linkend="ssl-tcp"> before using this. The default
        is off.  This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-renegotiation-limit" xreflabel="ssl_renegotiation_limit">
      <term><varname>ssl_renegotiation_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>ssl_renegotiation_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies how much data can flow over an <acronym>SSL</> encrypted connection
        before renegotiation of the session will take place. Renegotiation of the
        session decreases the chance of doing cryptanalysis when large amounts of data
        are sent, but it also carries a large performance penalty. The sum of
        sent and received traffic is used to check the limit. If the parameter is
        set to 0, renegotiation is disabled. The default is <literal>512MB</>.
       </para>
       <note>
        <para>
         SSL libraries from before November 2009 are insecure when using SSL
         renegotiation, due to a vulnerability in the SSL protocol. As a stop-gap fix
         for this vulnerability, some vendors also shipped SSL libraries incapable
         of doing renegotiation. If any of these libraries are in use on the client
         or server, SSL renegotiation should be disabled.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-password-encryption" xreflabel="password_encryption">
      <term><varname>password_encryption</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>password_encryption</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When a password is specified in <xref
        linkend="sql-createuser" endterm="sql-createuser-title"> or
        <xref linkend="sql-alteruser" endterm="sql-alteruser-title">
        without writing either <literal>ENCRYPTED</> or
        <literal>UNENCRYPTED</>, this option determines whether the
        password is to be encrypted. The default is on (encrypt the
        password).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
      <term><varname>krb_server_keyfile</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>krb_server_keyfile</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the location of the Kerberos server key file. See
        <xref linkend="kerberos-auth"> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
      <term><varname>db_user_namespace</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>db_user_namespace</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This allows per-database user names.  It is off by default.
       </para>

       <para>
        If this is on, you should create users as <literal>username@dbname</>.
        When <literal>username</> is passed by a connecting client,
        <literal>@</> and the database name is appended to the user
        name and that database-specific user name is looked up by the
        server. Note that when you create users with names containing
        <literal>@</> within the SQL environment, you will need to
        quote the user name.
       </para>

       <para>
        With this option enabled, you can still create ordinary global
        users.  Simply append <literal>@</> when specifying the user
        name in the client.  The <literal>@</> will be stripped off
        before the user name is looked up by the server.
       </para>

       <note>
        <para>
         This feature is intended as a temporary measure until a
         complete solution is found.  At that time, this option will
         be removed.
        </para>
       </note>
      </listitem>
     </varlistentry>

    </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-resource">
    <title>Resource Consumption</title>

    <sect3 id="runtime-config-resource-memory">
     <title>Memory</title>

     <variablelist>
     <varlistentry id="guc-shared-buffers" xreflabel="shared_buffers">
      <term><varname>shared_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>shared_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the number of shared memory buffers used by the database
        server. The default is typically 1000, but may be less if your
        kernel settings will not support it (as determined during
        <application>initdb</>).  Each buffer is 8192 bytes, unless a
        different value of <symbol>BLCKSZ</symbol> was chosen when building
        the server.  This setting must be at least 16, as well as at
        least twice the value of <xref linkend="guc-max-connections">;
        however, settings significantly higher than the minimum are
        usually needed for good performance.  Values of a few thousand
        are recommended for production installations.  This option can
        only be set at server start.
       </para>

       <para>
        Increasing this parameter may cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-work-mem" xreflabel="work_mem">
      <term><varname>work_mem</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>work_mem</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the amount of memory to be used by internal sort operations
        and hash tables before switching to temporary disk files. The value is
        specified in kilobytes, and defaults to 1024 kilobytes (1 MB).
        Note that for a complex query, several sort or hash operations might be
        running in parallel; each one will be allowed to use as much memory
        as this value specifies before it starts to put data into temporary
        files. Also, several running sessions could be doing such operations
        concurrently.  So the total memory used could be many
        times the value of <varname>work_mem</varname>; it is necessary to
        keep this fact in mind when choosing the value. Sort operations are
        used for <literal>ORDER BY</>, <literal>DISTINCT</>, and
        merge joins.
        Hash tables are used in hash joins, hash-based aggregation, and
        hash-based processing of <literal>IN</> subqueries.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-maintenance-work-mem" xreflabel="maintenance_work_mem">
      <term><varname>maintenance_work_mem</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>maintenance_work_mem</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum amount of memory to be used in maintenance
        operations, such as <command>VACUUM</command>, <command>CREATE
        INDEX</>, and <command>ALTER TABLE ADD FOREIGN KEY</>.
        The value is specified in kilobytes, and defaults to 16384 kilobytes
        (16 MB).  Since only one of these operations can be executed at 
        a time by a database session, and an installation normally doesn't
        have very many of them happening concurrently, it's safe to set this
        value significantly larger than <varname>work_mem</varname>.  Larger
        settings may improve performance for vacuuming and for restoring
        database dumps.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
      <term><varname>max_stack_depth</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_stack_depth</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum safe depth of the server's execution stack.
        The ideal setting for this parameter is the actual stack size limit
        enforced by the kernel (as set by <literal>ulimit -s</> or local
        equivalent), less a safety margin of a megabyte or so.  The safety
        margin is needed because the stack depth is not checked in every
        routine in the server, but only in key potentially-recursive routines
        such as expression evaluation.  Setting the parameter higher than
        the actual kernel limit will mean that a runaway recursive function
        can crash an individual backend process.  The default setting is
        2048 KB (two megabytes), which is conservatively small and unlikely
        to risk crashes.  However, it may be too small to allow execution
        of complex functions.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect3>
     <sect3 id="runtime-config-resource-fsm">
     <title>Free Space Map</title>

     <variablelist>
     <varlistentry id="guc-max-fsm-pages" xreflabel="max_fsm_pages">
      <term><varname>max_fsm_pages</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_fsm_pages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of disk pages for which free space will
        be tracked in the shared free-space map.  Six bytes of shared memory
        are consumed for each page slot.  This setting must be more than
        16 * <varname>max_fsm_relations</varname>.  The default is 20000.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-fsm-relations" xreflabel="max_fsm_relations">
      <term><varname>max_fsm_relations</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_fsm_relations</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of relations (tables and indexes) for which
        free space will be tracked in the shared free-space map.  Roughly
        fifty bytes of shared memory are consumed for each slot.
        The default is 1000.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-resource-kernel">
     <title>Kernel Resource Usage</title>
     <variablelist>

     <varlistentry id="guc-max-files-per-process" xreflabel="max_files_per_process">
      <term><varname>max_files_per_process</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_files_per_process</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of simultaneously open files allowed to each
        server subprocess. The default is 1000. If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can really support when a large number of processes all try to open
        that many files. If you find yourself seeing <quote>Too many open
        files</> failures, try reducing this setting.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-preload-libraries" xreflabel="preload_libraries">
      <term><varname>preload_libraries</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>preload_libraries</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are
        to be preloaded at server start. A parameterless
        initialization function can optionally be called for each
        library.  To specify that, add a colon and the name of the
        initialization function after the library name. For example
        <literal>'$libdir/mylib:mylib_init'</literal> would cause
        <literal>mylib</> to be preloaded and <literal>mylib_init</>
        to be executed. If more than one library is to be loaded,
        separate their names with commas.
       </para>

       <para>
        If a specified library or initialization function is not found,
        the server will fail to start.
       </para>

       <para>
        <productname>PostgreSQL</productname> procedural language
        libraries may be preloaded in this way, typically by using the
        syntax <literal>'$libdir/plXXX:plXXX_init'</literal> where
        <literal>XXX</literal> is <literal>pgsql</>, <literal>perl</>,
        <literal>tcl</>, or <literal>python</>.
       </para>

       <para>
        By preloading a shared library (and initializing it if
        applicable), the library startup time is avoided when the
        library is first used.  However, the time to start each new
        server process may increase slightly, even if that process never
        uses the library.  So this option is recommended only for
        libraries that will be used in most sessions.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>

    <sect3 id="runtime-config-resource-vacuum-cost">
     <title>Cost-Based Vacuum Delay</title>

     <para>
      During the execution of <xref linkend="sql-vacuum"
      endterm="sql-vacuum-title"> and <xref linkend="sql-analyze"
      endterm="sql-analyze-title"> commands, the system maintains an
      internal counter that keeps track of the estimated cost of the
      various I/O operations that are performed.  When the accumulated
      cost reaches a limit (specified by
      <varname>vacuum_cost_limit</varname>), the process performing
      the operation will sleep for a while (specified by
      <varname>vacuum_cost_delay</varname>). Then it will reset the
      counter and continue execution.
     </para>

     <para>
      The intent of this feature is to allow administrators to reduce
      the I/O impact of these commands on concurrent database
      activity. There are many situations in which it is not very
      important that maintenance commands like
      <command>VACUUM</command> and <command>ANALYZE</command> finish
      quickly; however, it is usually very important that these
      commands do not significantly interfere with the ability of the
      system to perform other database operations. Cost-based vacuum
      delay provides a way for administrators to achieve this.
     </para>

     <para>
      This feature is disabled by default. To enable it, set the
      <varname>vacuum_cost_delay</varname> variable to a nonzero
      value.
     </para>

     <variablelist>
      <varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
       <term><varname>vacuum_cost_delay</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_delay</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The length of time, in milliseconds, that the process will sleep
         when the cost limit has been exceeded.
         The default value is 0, which disables the cost-based vacuum
         delay feature.  Positive values enable cost-based vacuuming.
         Note that on many systems, the effective resolution
         of sleep delays is 10 milliseconds; setting
         <varname>vacuum_cost_delay</varname> to a value that is
         not a multiple of 10 may have the same results as setting it
         to the next higher multiple of 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
       <term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_hit</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer found in the shared buffer
         cache. It represents the cost to lock the buffer pool, lookup
         the shared hash table and scan the content of the page. The
         default value is 1.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
       <term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_miss</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer that has to be read from
         disk.  This represents the effort to lock the buffer pool,
         lookup the shared hash table, read the desired block in from
         the disk and scan its content. The default value is 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
       <term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_dirty</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost charged when vacuum modifies a block that was
         previously clean. It represents the extra I/O required to
         flush the dirty block out to disk again. The default value is
         20.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
       <term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_limit</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The accumulated cost that will cause the vacuuming process to sleep.
         The default value is 200.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <note>
      <para>
       There are certain operations that hold critical locks and should
       therefore complete as quickly as possible.  Cost-based vacuum
       delays do not occur during such operations.  Therefore it is
       possible that the cost accumulates far higher than the specified
       limit.  To avoid uselessly long delays in such cases, the actual
       delay is calculated as <varname>vacuum_cost_delay</varname> *
       <varname>accumulated_balance</varname> /
       <varname>vacuum_cost_limit</varname> with a maximum of
       <varname>vacuum_cost_delay</varname> * 4.
      </para>
     </note>

    </sect3>

    <sect3 id="runtime-config-resource-background-writer">
     <title>Background Writer</title>

     <para>
      Beginning in <productname>PostgreSQL</> 8.0, there is a separate server
      process called the <firstterm>background writer</>, whose sole function
      is to issue writes of <quote>dirty</> shared buffers.  The intent is
      that server processes handling user queries should seldom or never have
      to wait for a write to occur, because the background writer will do it.
      This arrangement also reduces the performance penalty associated with
      checkpoints.  The background writer will continuously trickle out dirty
      pages to disk, so that only a few pages will need to be forced out when
      checkpoint time arrives, instead of the storm of dirty-buffer writes that
      formerly occurred at each checkpoint.  However there is a net overall
      increase in I/O load, because where a repeatedly-dirtied page might
      before have been written only once per checkpoint interval, the
      background writer might write it several times in the same interval.
      In most situations a continuous low load is preferable to periodic
      spikes, but the parameters discussed in this section can be used to tune
      the behavior for local needs.
     </para>

     <variablelist>
      <varlistentry id="guc-bgwriter-delay" xreflabel="bgwriter_delay">
       <term><varname>bgwriter_delay</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_delay</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         Specifies the delay between activity rounds for the
         background writer.  In each round the writer issues writes
         for some number of dirty buffers (controllable by the
         following parameters).  The selected buffers will always be
         the least recently used ones among the currently dirty
         buffers.  It then sleeps for <varname>bgwriter_delay</>
         milliseconds, and repeats.  The default value is 200. Note
         that on many systems, the effective resolution of sleep
         delays is 10 milliseconds; setting <varname>bgwriter_delay</>
         to a value that is not a multiple of 10 may have the same
         results as setting it to the next higher multiple of 10.
         This option can only be set at server start or in the
         <filename>postgresql.conf</filename> file.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-percent" xreflabel="bgwriter_percent">
       <term><varname>bgwriter_percent</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_percent</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         In each round, no more than this percentage of the currently
         dirty buffers will be written (rounding up any fraction to
         the next whole number of buffers).  The default value is
         1. This option can only be set at server start or in the
         <filename>postgresql.conf</filename> file.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-maxpages" xreflabel="bgwriter_maxpages">
       <term><varname>bgwriter_maxpages</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_maxpages</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         In each round, no more than this many dirty buffers will be
         written. The default value is 100. This option can only be
         set at server start or in the
         <filename>postgresql.conf</filename> file.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Smaller values of <varname>bgwriter_percent</varname> and
      <varname>bgwriter_maxpages</varname> reduce the extra I/O load
      caused by the background writer, but leave more work to be done
      at checkpoint time.  To reduce load spikes at checkpoints,
      increase the values.  To disable background writing entirely,
      set <varname>bgwriter_percent</varname> and/or
      <varname>bgwriter_maxpages</varname> to zero.
     </para>
    </sect3>

   </sect2>

   <sect2 id="runtime-config-wal">
    <title>Write Ahead Log</title>

   <para>
    See also <xref linkend="wal-configuration"> for details on WAL
    tuning.
   </para>

    <sect3 id="runtime-config-wal-settings">
     <title>Settings</title>
     <variablelist>
     
     <varlistentry id="guc-fsync" xreflabel="fsync">
      <indexterm>
       <primary><varname>fsync</> configuration parameter</primary>
      </indexterm>
      <term><varname>fsync</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this option is on, the <productname>PostgreSQL</> server
        will use the <function>fsync()</> system call in several places
        to make sure that updates are physically written to disk. This
        insures that a database cluster will recover to a
        consistent state after an operating system or hardware crash.
       </para>

       <para>
        However, using <function>fsync()</function> results in a
        performance penalty: when a transaction is committed,
        <productname>PostgreSQL</productname> must wait for the
        operating system to flush the write-ahead log to disk.  When
        <varname>fsync</varname> is disabled, the operating system is
        allowed to do its best in buffering, ordering, and delaying
        writes. This can result in significantly improved performance.
        However, if the system crashes, the results of the last few
        committed transactions may be lost in part or whole. In the
        worst case, unrecoverable data corruption may occur.
        (Crashes of the database server itself are <emphasis>not</>
        a risk factor here.  Only an operating-system-level crash
        creates a risk of corruption.)
       </para>

       <para>
        Due to the risks involved, there is no universally correct
        setting for <varname>fsync</varname>. Some administrators
        always disable <varname>fsync</varname>, while others only
        turn it off for bulk loads, where there is a clear restart
        point if something goes wrong, whereas some administrators
        always leave <varname>fsync</varname> enabled. The default is
        to enable <varname>fsync</varname>, for maximum reliability.
        If you trust your operating system, your hardware, and your
        utility company (or your battery backup), you can consider
        disabling <varname>fsync</varname>.
       </para>

       <para>
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
      <term><varname>wal_sync_method</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>wal_sync_method</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Method used for forcing WAL updates out to disk.  Possible
        values are
        <literal>fsync</> (call <function>fsync()</> at each commit),
        <literal>fdatasync</> (call <function>fdatasync()</> at each commit),
        <literal>fsync_writethrough</> (call <function>_commit()</> at each commit on Windows),
        <literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>), and
        <literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>).
        Not all of these choices are available on all platforms.
        If <varname>fsync</varname> is off then this setting is irrelevant.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
      <term><varname>wal_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>wal_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Number of disk-page buffers allocated in shared memory for WAL data.
        The default is 8.  The setting need only be large enough to hold
        the amount of WAL data generated by one typical transaction.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
                
     <varlistentry id="guc-commit-delay" xreflabel="commit_delay">
      <term><varname>commit_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>commit_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Time delay between writing a commit record to the WAL buffer
        and flushing the buffer out to disk, in microseconds. A
        nonzero delay can allow multiple transactions to be committed
        with only one <function>fsync()</function> system call, if
        system load is high enough that additional transactions become
        ready to commit within the given interval. But the delay is
        just wasted if no other transactions become ready to
        commit. Therefore, the delay is only performed if at least
        <varname>commit_siblings</varname> other transactions are
        active at the instant that a server process has written its
        commit record. The default is zero (no delay).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
      <term><varname>commit_siblings</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>commit_siblings</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Minimum number of concurrent open transactions to require
        before performing the <varname>commit_delay</> delay. A larger
        value makes it more probable that at least one other
        transaction will become ready to commit during the delay
        interval. The default is five.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect3>
     <sect3 id="runtime-config-wal-checkpoints">
     <title>Checkpoints</title>

    <variablelist>
     <varlistentry id="guc-checkpoint-segments" xreflabel="checkpoint_segments">
      <term><varname>checkpoint_segments</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_segments</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Maximum distance between automatic WAL checkpoints, in log
        file segments (each segment is normally 16 megabytes). The
        default is three.  This option can only be set at server start
        or in the <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-timeout" xreflabel="checkpoint_timeout">
      <term><varname>checkpoint_timeout</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_timeout</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Maximum time between automatic WAL checkpoints, in
        seconds. The default is 300 seconds.  This option can only be
        set at server start or in the <filename>postgresql.conf</>
        file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-warning" xreflabel="checkpoint_warning">
      <term><varname>checkpoint_warning</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_warning</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Write a message to the server log if checkpoints caused by
        the filling of checkpoint segment files happen closer together
        than this many seconds.  The default is 30 seconds.
        Zero turns off the warning.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect3>
     <sect3 id="runtime-config-wal-archiving">
     <title>Archiving</title>

    <variablelist>
     <varlistentry id="guc-archive-command" xreflabel="archive_command">
      <term><varname>archive_command</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>archive_command</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The shell command to execute to archive a completed segment of
        the WAL file series. If this is an empty string (the default),
        WAL archiving is disabled. Any <literal>%p</> in the string is
        replaced by the absolute path of the file to archive, and any
        <literal>%f</> is replaced by the file name only. Use
        <literal>%%</> to embed an actual <literal>%</> character in the
        command. For more information see <xref
        linkend="backup-archiving-wal">. This option can only be set at
        server start or in the <filename>postgresql.conf</filename>
        file.
       </para>
       <para>
        It is important for the command to return a zero exit status if
        and only if it succeeds.  Examples:
<programlisting>
archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" /mnt/server/archivedir/"%f"'  # Windows
</programlisting>
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-query">
    <title>Query Planning</title>

    <sect3 id="runtime-config-query-enable">
     <title>Planner Method Configuration</title>

      <para>
       These configuration parameters provide a crude method of
       influencing the query plans chosen by the query optimizer. If
       the default plan chosen by the optimizer for a particular query
       is not optimal, a temporary solution may be found by using one
       of these configuration parameters to force the optimizer to
       choose a different plan.  Turning one of these settings off
       permanently is seldom a good idea, however.
       Better ways to improve the quality of the
       plans chosen by the optimizer include adjusting the <xref
       linkend="runtime-config-query-constants"
       endterm="runtime-config-query-constants-title">, running <xref
       linkend="sql-analyze" endterm="sql-analyze-title"> more
       frequently, increasing the value of the <xref
       linkend="guc-default-statistics-target"> configuration parameter,
       and increasing the amount of statistics collected for
       specific columns using <command>ALTER TABLE SET
       STATISTICS</command>.
      </para>

     <variablelist>
     <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
      <term><varname>enable_hashagg</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_hashagg</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of hashed
        aggregation plan types. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-hashjoin" xreflabel="enable_hashjoin">
      <term><varname>enable_hashjoin</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_hashjoin</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of hash-join plan
        types. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-indexscan" xreflabel="enable_indexscan">
      <term><varname>enable_indexscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>index scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_indexscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of index-scan plan
        types. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
      <term><varname>enable_mergejoin</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_mergejoin</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of merge-join plan
        types. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-nestloop" xreflabel="enable_nestloop">
      <term><varname>enable_nestloop</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_nestloop</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of nested-loop join
        plans. It's not possible to suppress nested-loop joins entirely,
        but turning this variable off discourages the planner from using
        one if there are other methods available. The default is
        on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
      <term><varname>enable_seqscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>sequential scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_seqscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of sequential scan
        plan types. It's not possible to suppress sequential scans
        entirely, but turning this variable off discourages the planner
        from using one if there are other methods available. The
        default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-sort" xreflabel="enable_sort">
      <term><varname>enable_sort</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_sort</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of explicit sort
        steps. It's not possible to suppress explicit sorts entirely,
        but turning this variable off discourages the planner from
        using one if there are other methods available. The default
        is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
      <term><varname>enable_tidscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_tidscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of <acronym>TID</>
        scan plan types. The default is on.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-query-constants">
     <title id="runtime-config-query-constants-title">
      Planner Cost Constants
     </title>

   <note>
    <para>
     Unfortunately, there is no well-defined method for determining
     ideal values for the family of <quote>cost</quote> variables that
     appear below. You are encouraged to experiment and share
     your findings.
    </para>
   </note>

     <variablelist>
     
     <varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
      <term><varname>effective_cache_size</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>effective_cache_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's assumption about the effective size of the
        disk cache that is available to a single index scan.  This is
        factored into estimates of the cost of using an index; a higher
        value makes it more likely index scans will be used, a lower
        value makes it more likely sequential scans will be used. When
        setting this parameter you should consider both
        <productname>PostgreSQL</productname>'s shared buffers and the
        portion of the kernel's disk cache that will be used for
        <productname>PostgreSQL</productname> data files.  Also, take into
        account the expected number of concurrent queries using different
        indexes, since they will have to share the available space.
        This parameter has no effect on the size of shared memory
        allocated by PostgreSQL, nor does it reserve kernel disk cache;
        it is used only for estimation purposes.
        The value is measured in disk pages, which are
        normally 8192 bytes each. The default is 1000.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
      <term><varname>random_page_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>random_page_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a
        nonsequentially fetched disk page. This is measured as a
        multiple of the cost of a sequential page fetch. A higher
        value makes it more likely a sequential scan will be used, a
        lower value makes it more likely an index scan will be
        used. The default is four.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-tuple-cost" xreflabel="cpu_tuple_cost">
      <term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_tuple_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each row during a query. This is measured as a fraction of
        the cost of a sequential page fetch. The default is 0.01.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-index-tuple-cost" xreflabel="cpu_index_tuple_cost">
      <term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_index_tuple_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each index row during an index scan. This is measured as a
        fraction of the cost of a sequential page fetch. The default
        is 0.001.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry id="guc-cpu-operator-cost" xreflabel="cpu_operator_cost">
      <term><varname>cpu_operator_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_operator_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing each
        operator in a <literal>WHERE</> clause. This is measured as a fraction of
        the cost of a sequential page fetch. The default is 0.0025.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
     <sect3 id="runtime-config-query-geqo">
     <title>Genetic Query Optimizer</title>

     <variablelist>

     <varlistentry id="guc-geqo" xreflabel="geqo">
      <indexterm>
       <primary>genetic query optimization</primary>
      </indexterm>
      <indexterm>
       <primary>GEQO</primary>
       <see>genetic query optimization</see>
      </indexterm>
      <indexterm>
       <primary><varname>geqo</> configuration parameter</primary>
      </indexterm>
      <term><varname>geqo</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables genetic query optimization, which is an
        algorithm that attempts to do query planning without
        exhaustive searching. This is on by default. The
        <varname>geqo_threshold</varname> variable provides a more
        granular way to disable GEQO for certain classes of queries.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
      <term><varname>geqo_threshold</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_threshold</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Use genetic query optimization to plan queries with at least
        this many <literal>FROM</> items involved. (Note that an outer
        <literal>JOIN</> construct counts as only one <literal>FROM</>
        item.) The default is 12. For simpler queries it is usually best
        to use the deterministic, exhaustive planner, but for queries with
        many tables the deterministic planner takes too long.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
      <term><varname>geqo_effort</varname>
      (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_effort</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the trade off between planning time and query plan
        efficiency in GEQO. This variable must be an integer in the
        range from 1 to 10. The default value is 5. Larger values
        increase the time spent doing query planning, but also
        increase the likelihood that an efficient query plan will be
        chosen.
       </para>

       <para>
        <varname>geqo_effort</varname> doesn't actually do anything
        directly; it is only used to compute the default values for
        the other variables that influence GEQO behavior (described
        below). If you prefer, you can set the other parameters by
        hand instead.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
      <term><varname>geqo_pool_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_pool_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the pool size used by GEQO. The pool size is the
        number of individuals in the genetic population.  It must be
        at least two, and useful values are typically 100 to 1000.  If
        it is set to zero (the default setting) then a suitable
        default is chosen based on <varname>geqo_effort</varname> and
        the number of tables in the query.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-generations" xreflabel="geqo_generations">
      <term><varname>geqo_generations</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_generations</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the number of generations used by GEQO.  Generations
        specifies the number of iterations of the algorithm.  It must
        be at least one, and useful values are in the same range as
        the pool size.  If it is set to zero (the default setting)
        then a suitable default is chosen based on
        <varname>geqo_pool_size</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-selection-bias" xreflabel="geqo_selection_bias">
      <term><varname>geqo_selection_bias</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>geqo_selection_bias</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the selection bias used by GEQO. The selection bias
        is the selective pressure within the population. Values can be
        from 1.50 to 2.00; the latter is the default.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
     <sect3 id="runtime-config-query-other">
     <title>Other Planner Options</title>

     <variablelist>

     <varlistentry id="guc-default-statistics-target" xreflabel="default_statistics_target">
      <term><varname>default_statistics_target</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>default_statistics_target</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the default statistics target for table columns that have
        not had a column-specific target set via <command>ALTER TABLE
        SET STATISTICS</>.  Larger values increase the time needed to
        do <command>ANALYZE</>, but may improve the quality of the
        planner's estimates. The default is 10. For more information
        on the use of statistics by the <productname>PostgreSQL</>
        query planner, refer to <xref linkend="planner-stats">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
      <term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>from_collapse_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The planner will merge sub-queries into upper queries if the
        resulting <literal>FROM</literal> list would have no more than
        this many items.  Smaller values reduce planning time but may
        yield inferior query plans.  The default is 8.  It is usually
        wise to keep this less than <xref linkend="guc-geqo-threshold">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
      <term><varname>join_collapse_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>join_collapse_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The planner will rewrite explicit inner <literal>JOIN</>
        constructs into lists of <literal>FROM</> items whenever a
        list of no more than this many items in total would
        result. Prior to <productname>PostgreSQL</> 7.4, joins
        specified via the <literal>JOIN</literal> construct would
        never be reordered by the query planner. The query planner has
        subsequently been improved so that inner joins written in this
        form can be reordered; this configuration parameter controls
        the extent to which this reordering is performed.
        <note>
         <para>
          At present, the order of outer joins specified via the
          <literal>JOIN</> construct is never adjusted by the query
          planner; therefore, <varname>join_collapse_limit</> has no
          effect on this behavior. The planner may be improved to
          reorder some classes of outer joins in a future release of
          <productname>PostgreSQL</productname>.
         </para>
        </note>
       </para>

       <para>
        By default, this variable is set the same as
        <varname>from_collapse_limit</varname>, which is appropriate
        for most uses. Setting it to 1 prevents any reordering of
        inner <literal>JOIN</>s. Thus, the explicit join order
        specified in the query will be the actual order in which the
        relations are joined. The query planner does not always choose
        the optimal join order; advanced users may elect to
        temporarily set this variable to 1, and then specify the join
        order they desire explicitly. Another consequence of setting
        this variable to 1 is that the query planner will behave more
        like the <productname>PostgreSQL</productname> 7.3 query
        planner, which some users might find useful for backward
        compatibility reasons.
       </para>

       <para>
        Setting this variable to a value between 1 and
        <varname>from_collapse_limit</varname> might be useful to
        trade off planning time against the quality of the chosen plan
        (higher values produce better plans).
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-logging">
    <title>Error Reporting and Logging</title>

    <indexterm zone="runtime-config-logging">
     <primary>server log</primary>
    </indexterm>

    <sect3 id="runtime-config-logging-where">
     <title>Where to log</title>

     <indexterm zone="runtime-config-logging-where">
      <primary>where to log</primary>
     </indexterm>

     <variablelist>

     <varlistentry id="guc-log-destination" xreflabel="log_destination">
      <term><varname>log_destination</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_destination</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        <productname>PostgreSQL</productname> supports several methods
         for logging server messages, including
         <systemitem>stderr</systemitem> and
         <systemitem>syslog</systemitem>. On Windows, 
         <systemitem>eventlog</systemitem> is also supported. Set this
         option to a list of desired log destinations separated by
         commas. The default is to log to <systemitem>stderr</systemitem> 
         only.
         This option can only be set at server start or in the
         <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-redirect-stderr" xreflabel="redirect_stderr">
      <term><varname>redirect_stderr</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>redirect_stderr</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         This option allows messages sent to <application>stderr</> to be
         captured and redirected into log files.
         This option, in combination with logging to <application>stderr</>,
         is often more useful than
         logging to <application>syslog</>, since some types of messages
         may not appear in <application>syslog</> output (a common example
         is dynamic-linker failure messages).
         This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-directory" xreflabel="log_directory">
      <term><varname>log_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>redirect_stderr</> is enabled, this option
        determines the directory in which log files will be created.
        It may be specified as an absolute path, or relative to the
        cluster data directory.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-filename" xreflabel="log_filename">
      <term><varname>log_filename</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_filename</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>redirect_stderr</varname> is enabled, this option
        sets the file names of the created log files.  The value
        is treated as a <systemitem>strftime</systemitem> pattern,
        so <literal>%</literal>-escapes
        can be used to specify time-varying file names.
        If no <literal>%</literal>-escapes are present,
        <productname>PostgreSQL</productname> will
        append the epoch of the new log file's open time.  For example,
        if <varname>log_filename</varname> were <literal>server_log</literal>, then the
        chosen file name would be <literal>server_log.1093827753</literal>
        for a log starting at Sun Aug 29 19:02:33 2004 MST.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-age" xreflabel="log_rotation_age">
      <term><varname>log_rotation_age</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_rotation_age</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>redirect_stderr</varname> is enabled, this option
        determines the maximum lifetime of an individual log file.
        After this many minutes have elapsed, a new log file will
        be created.  Set to zero to disable time-based creation of
        new log files.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-size" xreflabel="log_rotation_size">
      <term><varname>log_rotation_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_rotation_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>redirect_stderr</varname> is enabled, this option
        determines the maximum size of an individual log file.
        After this many kilobytes have been emitted into a log file,
        a new log file will be created.  Set to zero to disable size-based
        creation of new log files.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-truncate-on-rotation" xreflabel="log_truncate_on_rotation">
      <term><varname>log_truncate_on_rotation</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_truncate_on_rotation</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>redirect_stderr</varname> is enabled, this option will cause
        <productname>PostgreSQL</productname> to truncate (overwrite),
        rather than append to, any existing log file of the same name.
        However, truncation will occur only when a new file is being opened
        due to time-based rotation, not during server startup or size-based
        rotation.  When false, pre-existing files will be appended to in
        all cases.  For example, using this option in combination with
        a <varname>log_filename</varname> like <literal>postgresql-%H.log</literal>
        would result in generating twenty-four hourly log files and then
        cyclically overwriting them.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
       <para>
        Example:  To keep 7 days of logs, one log file per day named
        <literal>server_log.Mon</literal>, <literal>server_log.Tue</literal>, 
        etc, and automatically overwrite last week's log with this week's log,
        set <varname>log_filename</varname> to <literal>server_log.%a</literal>, 
        <varname>log_truncate_on_rotation</varname> to <literal>true</literal>, and 
        <varname>log_rotation_age</varname> to <literal>1440</literal>.
       </para>
       <para>
        Example: To keep 24 hours of logs, one log file per hour, but 
        also rotate sooner if the log file size exceeds 1GB, set 
        <varname>log_filename</varname> to <literal>server_log.%H%M</literal>, 
        <varname>log_truncate_on_rotation</varname> to <literal>true</literal>, 
        <varname>log_rotation_age</varname> to <literal>60</literal>, and 
        <varname>log_rotation_size</varname> to <literal>1000000</literal>.
        Including <literal>%M</> in <varname>log_filename</varname> allows
        any size-driven rotations that may occur to select a filename
        different from the hour's initial filename.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-syslog-facility" xreflabel="syslog_facility">
      <term><varname>syslog_facility</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>syslog_facility</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When logging to <application>syslog</> is enabled, this option
        determines the <application>syslog</application>
        <quote>facility</quote> to be used.  You may choose
        from <literal>LOCAL0</>, <literal>LOCAL1</>,
        <literal>LOCAL2</>, <literal>LOCAL3</>, <literal>LOCAL4</>,
        <literal>LOCAL5</>, <literal>LOCAL6</>, <literal>LOCAL7</>;
        the default is <literal>LOCAL0</>. See also the
        documentation of your system's
        <application>syslog</application> daemon.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-syslog-ident" xreflabel="syslog_ident">
      <term><varname>syslog_ident</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>syslog_identity</> configuration parameter</primary>
      </indexterm>
       <listitem>
        <para>
         When logging to <application>syslog</> is enabled, this option
         determines the program name used to identify
         <productname>PostgreSQL</productname> messages in
         <application>syslog</application> logs. The default is
         <literal>postgres</literal>.
          This option can only be set at server start.
        </para>
       </listitem>
      </varlistentry>
      
      </variablelist>
    </sect3>
     <sect3 id="runtime-config-logging-when">
     <title>When To Log</title>

     <variablelist>

     <varlistentry id="guc-client-min-messages" xreflabel="client_min_messages">
      <term><varname>client_min_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>client_min_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which message levels are sent to the client.
        Valid values are <literal>DEBUG5</>,
        <literal>DEBUG4</>, <literal>DEBUG3</>, <literal>DEBUG2</>,
        <literal>DEBUG1</>, <literal>LOG</>, <literal>NOTICE</>,
        <literal>WARNING</>, and <literal>ERROR</>.  Each level
        includes all the levels that follow it.  The later the level,
        the fewer messages are sent.  The default is
        <literal>NOTICE</>.  Note that <literal>LOG</> has a different
        rank here than in <varname>log_min_messages</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-messages" xreflabel="log_min_messages">
      <term><varname>log_min_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_min_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which message levels are written to the server log.
        Valid values are <literal>DEBUG5</>, <literal>DEBUG4</>,
        <literal>DEBUG3</>, <literal>DEBUG2</>, <literal>DEBUG1</>,
        <literal>INFO</>, <literal>NOTICE</>, <literal>WARNING</>,
        <literal>ERROR</>, <literal>LOG</>, <literal>FATAL</>, and
        <literal>PANIC</>.  Each level includes all the levels that
        follow it.  The later the level, the fewer messages are sent
        to the log.  The default is <literal>NOTICE</>.  Note that
        <literal>LOG</> has a different rank here than in
        <varname>client_min_messages</>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-error-verbosity" xreflabel="log_error_verbosity">
      <term><varname>log_error_verbosity</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_error_verbosity</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the amount of detail written in the server log for each
        message that is logged.  Valid values are <literal>TERSE</>,
        <literal>DEFAULT</>, and <literal>VERBOSE</>, each adding more
        fields to displayed messages.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-error-statement" xreflabel="log_min_error_statement">
      <term><varname>log_min_error_statement</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_min_error_statement</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether or not the SQL statement that causes an error
        condition will also be recorded in the server log. All SQL
        statements that cause an error of the specified level or
        higher are logged.  The default is
        <literal>PANIC</literal> (effectively turning this feature
        off for normal use). Valid values are <literal>DEBUG5</literal>,
        <literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
        <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
        <literal>INFO</literal>, <literal>NOTICE</literal>,
        <literal>WARNING</literal>, <literal>ERROR</literal>,
        <literal>FATAL</literal>, and <literal>PANIC</literal>.  For
        example, if you set this to <literal>ERROR</literal> then all
        SQL statements causing errors, fatal errors, or panics will be
        logged. Enabling this option can be helpful in tracking down
        the source of any errors that appear in the server log.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-log-min-duration-statement" xreflabel="log_min_duration_statement">
      <term><varname>log_min_duration_statement</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_min_duration_statement</> configuration parameter</primary>
      </indexterm>
       <listitem>
        <para>
         Sets a minimum statement execution time (in milliseconds)
         that causes a statement to be logged.  All SQL statements
         that run for the time specified or longer will be logged with
         their duration.  Setting this to zero will print
         all queries and their durations.  Minus-one (the default)
         disables the feature.  For example, if you set it to 
         <literal>250</literal> then all SQL statements that run 250ms 
         or longer will be logged.  Enabling this option can be
         useful in tracking down unoptimized queries in your applications.
         Only superusers can change this setting.
        </para>
       </listitem>
      </varlistentry>

     <varlistentry id="guc-silent-mode" xreflabel="silent_mode">
      <term><varname>silent_mode</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>silent_mode</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Runs the server silently. If this option is set, the server
        will automatically run in background and any controlling
        terminals are disassociated (same effect as
        <command>postmaster</>'s <option>-S</option> option).
        The server's standard output and standard error are redirected
        to <literal>/dev/null</>, so any messages sent to them will be lost.
        Unless <application>syslog</> logging is selected or
        <varname>redirect_stderr</> is enabled, using this option
        is discouraged because it makes it impossible to see error messages.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

       <para>
        Here is a list of the various message severity levels used in
        these settings:
        <variablelist>
         <varlistentry>
          <term><literal>DEBUG[1-5]</literal></term>
          <listitem>
           <para>
            Provides information for use by developers.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>INFO</literal></term>
          <listitem>
           <para>
            Provides information implicitly requested by the user,
            e.g., during <command>VACUUM VERBOSE</>.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>NOTICE</literal></term>
          <listitem>
           <para>
            Provides information that may be helpful to users, e.g.,
            truncation of long identifiers and the creation of indexes as part
            of primary keys.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>WARNING</literal></term>
          <listitem>
           <para>
            Provides warnings to the user, e.g., <command>COMMIT</>
            outside a transaction block.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>ERROR</literal></term>
          <listitem>
           <para>
            Reports an error that caused the current command to abort.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>LOG</literal></term>
          <listitem>
           <para>
            Reports information of interest to administrators, e.g.,
            checkpoint activity.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>FATAL</literal></term>
          <listitem>
           <para>
            Reports an error that caused the current session to abort.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PANIC</literal></term>
          <listitem>
           <para>
            Reports an error that caused all sessions to abort.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

    </sect3>
     <sect3 id="runtime-config-logging-what">
     <title>What To Log</title>

     <variablelist>

     <varlistentry>
      <term><varname>debug_print_parse</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_rewritten</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_plan</varname> (<type>boolean</type>)</term>
      <term><varname>debug_pretty_print</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>debug_print_parse</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_print_rewritten</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_print_plan</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_pretty_print</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        These options enable various debugging output to be emitted.
        For each executed query, they print
        the resulting parse tree, the query rewriter output, or the
        execution plan.  <varname>debug_pretty_print</varname> indents
        these displays to produce a more readable but much longer
        output format.  <varname>client_min_messages</varname> or
        <varname>log_min_messages</varname> must be
        <literal>DEBUG1</literal> or lower to actually send this output
        to the client or the server log, respectively.
        These options are off by default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-connections" xreflabel="log_connections">
      <term><varname>log_connections</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This outputs a line to the server log detailing each successful
        connection. This is off by default, although it is probably very
        useful. This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-disconnections" xreflabel="log_disconnections">
      <term><varname>log_disconnections</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_disconnections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This outputs a line in the server log similar to
        <varname>log_connections</varname> but at session termination,
        and includes the duration of the session.  This is off by
        default. This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry id="guc-log-duration" xreflabel="log_duration">
      <term><varname>log_duration</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_duration</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Causes the duration of every completed statement which satisfies
        <varname>log_statement</> to be logged.  When using this option, 
        if you are not using <application>syslog</>, it is recommended 
        that you log the PID or session ID using <varname>log_line_prefix</> 
        so that you can link the statement to the 
        duration using the process ID or session ID. The default is off.  
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-log-line-prefix" xreflabel="log_line_prefix">
      <term><varname>log_line_prefix</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_line_prefix</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         This is a <function>printf</>-style string that is output at the
         beginning of each log line. The default is an empty string.
         Each recognized escape is replaced as outlined 
         below - anything else that looks like an escape is ignored. Other
         characters are copied straight to the log line. Some escapes are
         only recognised by session processes, and do not apply to
         background processes such as the postmaster. <application>Syslog</>
         produces its own 
         time stamp and process ID information, so you probably do not want to
         use those escapes if you are using <application>syslog</>.
         This option can only be set at server start or in the
         <filename>postgresql.conf</filename> configuration file.

         <informaltable>
          <tgroup cols="3">
           <thead>
            <row>
             <entry>Escape</entry>
             <entry>Effect</entry>
             <entry>Session only</entry>
             </row>
            </thead>
           <tbody>
            <row>
             <entry><literal>%u</literal></entry>
             <entry>User name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%d</literal></entry>
             <entry>Database name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%r</literal></entry>
             <entry>Remote host name or IP address, and remote port</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%p</literal></entry>
             <entry>Process ID</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%t</literal></entry>
             <entry>Time stamp</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%i</literal></entry>
             <entry>Command tag: This is the command that generated the log line.</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%c</literal></entry>
             <entry>Session ID: A unique identifier for each session.
             It is 2 4-byte hexadecimal numbers (without leading zeros) 
             separated by a dot. The numbers
             are the session start time and the process ID, so this can also
             be used as a space saving way of printing these items.</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%l</literal></entry>
             <entry>Number of the log line for each process, starting at 1</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%s</literal></entry>
             <entry>Session start time stamp</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%x</literal></entry>
             <entry>Transaction ID</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%q</literal></entry>
             <entry>Does not produce any output, but tells non-session
             processes to stop at this point in the string. Ignored by
             session processes.</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%%</literal></entry>
             <entry>Literal <literal>%</></entry>
             <entry>no</entry>
            </row>
           </tbody>
          </tgroup>
         </informaltable>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-statement" xreflabel="log_statement">
      <term><varname>log_statement</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_statement</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which SQL statements are logged. Valid values are
        <literal>none</>, <literal>ddl</>, <literal>mod</>, and
        <literal>all</>. <literal>ddl</> logs all data definition
        commands like <literal>CREATE</>, <literal>ALTER</>, and
        <literal>DROP</> commands. <literal>mod</> logs all
        <literal>ddl</> statements, plus <literal>INSERT</>,
        <literal>UPDATE</>, <literal>DELETE</>, <literal>TRUNCATE</>,
        and <literal>COPY FROM</>. <literal>PREPARE</> and
        <literal>EXPLAIN ANALYZE</> statements are also logged if their
        contained command is of an appropriate type.
       </para>
       <para>
        The default is <literal>none</>. Only superusers can change this
        setting.
       </para>

       <note>
        <para>
         The <command>EXECUTE</command> statement is not considered a
         <literal>ddl</> or <literal>mod</> statement.  When it is logged, 
         only the name of the prepared statement is reported, not the
         actual prepared statement.
        </para>

        <para>
         When a function is defined in the
         <application>PL/pgSQL</application>server-side language, any queries
         executed by the function will only be logged the first time that the
         function is invoked in a particular session. This is because
         <application>PL/pgSQL</application> keeps a cache of the
         query plans produced for the SQL statements in the function.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-hostname" xreflabel="log_hostname">
      <term><varname>log_hostname</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_hostname</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        By default, connection log messages only show the IP address of the
        connecting host. Turning on this option causes logging of the
        host name as well.  Note that depending on your host name resolution
        setup this might impose a non-negligible performance penalty. This
        option can only be set at server start or in the
        <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-statistics">
    <title>Runtime Statistics</title>

    <sect3 id="runtime-config-statistics-monitor">
     <title>Statistics Monitoring</title>
     <variablelist>

     <varlistentry>
      <term><varname>log_statement_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_parser_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_planner_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_executor_stats</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_statement_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_parser_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_planner_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_executor_stats</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        For each query, write performance statistics of the respective
        module to the server log. This is a crude profiling
        instrument.  <varname>log_statement_stats</varname> reports total
        statement statistics, while the others report per-module statistics.
        <varname>log_statement_stats</varname> cannot be enabled together with
        any of the per-module options.  All of these options are disabled by
        default.   Only superusers can change these settings.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
    <sect3 id="runtime-config-statistics-collector">
     <title>Query and Index Statistics Collector</title>
     <variablelist>

     <varlistentry id="guc-stats-start-collector" xreflabel="stats_start_collector">
      <term><varname>stats_start_collector</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>stats_start_collector</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether the server should start the
        statistics-collection subprocess.  This is on by default, but
        may be turned off if you know you have no interest in
        collecting statistics.  This option can only be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-stats-command-string" xreflabel="stats_command_string">
      <term><varname>stats_command_string</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>stats_command_string</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables the collection of statistics on the currently
        executing command of each session, along with the time at
        which that command began execution. This option is off by
        default. Note that even when enabled, this information is not
        visible to all users, only to superusers and the user owning
        the session being reported on; so it should not represent a
        security risk. This data can be accessed via the
        <structname>pg_stat_activity</structname> system view; refer
        to <xref linkend="monitoring"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-stats-block-level" xreflabel="stats_block_level">
      <term><varname>stats_block_level</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>stats_block_level</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables the collection of block-level statistics on database
        activity. This option is disabled by default. If this option
        is enabled, the data that is produced can be accessed via the
        <structname>pg_stat</structname> and
        <structname>pg_statio</structname> family of system views;
        refer to <xref linkend="monitoring"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-stats-row-level" xreflabel="stats_row_level">
      <term><varname>stats_row_level</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>stats_row_level</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables the collection of row-level statistics on database
        activity. This option is disabled by default. If this option
        is enabled, the data that is produced can be accessed via the
        <structname>pg_stat</structname> and
        <structname>pg_statio</structname> family of system views;
        refer to <xref linkend="monitoring"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-stats-reset-on-server-start" xreflabel="stats_reset_on_server_start">
      <term><varname>stats_reset_on_server_start</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>stats_reset_on_server_start</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If on, collected statistics are zeroed out whenever the server
        is restarted. If off, statistics are accumulated across server
        restarts. The default is on. This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-client">
    <title>Client Connection Defaults</title>

    <sect3 id="runtime-config-client-statement">
     <title>Statement Behavior</title>
     <variablelist>

     <varlistentry id="guc-search-path" xreflabel="search_path">
      <term><varname>search_path</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>search_path</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>path</><secondary>for schemas</></>
      <listitem>
       <para>
        This variable specifies the order in which schemas are searched
        when an object (table, data type, function, etc.) is referenced by a
        simple name with no schema component.  When there are objects of
        identical names in different schemas, the one found first
        in the search path is used.  An object that is not in any of the
        schemas in the search path can only be referenced by specifying
        its containing schema with a qualified (dotted) name.
       </para>

       <para>
        The value for <varname>search_path</varname> has to be a comma-separated
        list of schema names.  If one of the list items is
        the special value <literal>$user</literal>, then the schema
        having the name returned by <function>SESSION_USER</> is substituted, if there
        is such a schema.  (If not, <literal>$user</literal> is ignored.)
       </para>

       <para>
        The system catalog schema, <literal>pg_catalog</>, is always
        searched, whether it is mentioned in the path or not.  If it is
        mentioned in the path then it will be searched in the specified
        order.  If <literal>pg_catalog</> is not in the path then it will
        be searched <emphasis>before</> searching any of the path items.
       </para>

       <para>
        Likewise, the current session's temporary-table schema,
        <literal>pg_temp_<replaceable>nnn</></>, is always searched if it
        exists.  It can be explicitly listed in the path by using the
        alias <literal>pg_temp</>.  If it is not listed in the path then
        it is searched first (before even <literal>pg_catalog</>).  However,
        the temporary schema is only searched for relation (table, view,
        sequence, etc) and data type names.  It will never be searched for
        function or operator names.
       </para>

       <para>
        When objects are created without specifying a particular target
        schema, they will be placed in the first schema listed
        in the search path.  An error is reported if the search path is
        empty.
       </para>

       <para>
        The default value for this parameter is
        <literal>'$user, public'</literal> (where the second part will be
        ignored if there is no schema named <literal>public</>).
        This supports shared use of a database (where no users
        have private schemas, and all share use of <literal>public</>),
        private per-user schemas, and combinations of these.  Other
        effects can be obtained by altering the default search path
        setting, either globally or per-user.
       </para>

       <para>
        The current effective value of the search path can be examined
        via the <acronym>SQL</acronym> function
        <function>current_schemas()</>.  This is not quite the same as
        examining the value of <varname>search_path</varname>, since
        <function>current_schemas()</> shows how the requests
        appearing in <varname>search_path</varname> were resolved.
       </para>

       <para>
        For more information on schema handling, see <xref linkend="ddl-schemas">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-tablespace" xreflabel="default_tablespace">
      <term><varname>default_tablespace</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>default_tablespace</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>tablespace</><secondary>default</></>
      <listitem>
       <para>
        This variable specifies the default tablespace in which to create
        objects (tables and indexes) when a <command>CREATE</> command does
        not explicitly specify a tablespace.
       </para>

       <para>
        The value is either the name of a tablespace, or an empty string
        to specify using the default tablespace of the current database.
        If the value does not match the name of any existing tablespace,
        <productname>PostgreSQL</> will automatically use the default
        tablespace of the current database.
       </para>

       <para>
        For more information on tablespaces,
        see <xref linkend="manage-ag-tablespaces">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-check-function-bodies" xreflabel="check_function_bodies">
      <term><varname>check_function_bodies</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>check_function_bodies</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This parameter is normally true.  When set to false, it disables
        validation of the function body string during <xref
        linkend="sql-createfunction"
        endterm="sql-createfunction-title">.  Disabling validation is
        occasionally useful to avoid problems such as forward
        references when restoring function definitions from a dump.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
      <indexterm>
       <primary>transaction isolation level</primary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_isolation</> configuration parameter</primary>
      </indexterm>
      <term><varname>default_transaction_isolation</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Each SQL transaction has an isolation level, which can be
        either <quote>read uncommitted</quote>, <quote>read
        committed</quote>, <quote>repeatable read</quote>, or
        <quote>serializable</quote>.  This parameter controls the
        default isolation level of each new transaction. The default
        is <quote>read committed</quote>.
       </para>

       <para>
        Consult <xref linkend="mvcc"> and <xref linkend="sql-set-transaction"> for more
        information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
      <indexterm>
       <primary>read-only transaction</primary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_read_only</> configuration parameter</primary>
      </indexterm>

      <term><varname>default_transaction_read_only</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        A read-only SQL transaction cannot alter non-temporary tables.
        This parameter controls the default read-only status of each new
        transaction. The default is false (read/write).
       </para>

       <para>
        Consult <xref linkend="sql-set-transaction"> for more information.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
      <term><varname>statement_timeout</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>statement_timeout</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Abort any statement that takes over the specified number of
        milliseconds.  A value of zero (the default) turns off the limitation.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
     <sect3 id="runtime-config-client-format">
     <title>Locale and Formatting</title>

     <variablelist>

     <varlistentry id="guc-datestyle" xreflabel="DateStyle">
      <term><varname>DateStyle</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>DateStyle</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the display format for date and time values, as well as the
        rules for interpreting ambiguous date input values. For
        historical reasons, this variable contains two independent
        components: the output format specification (<literal>ISO</>,
        <literal>Postgres</>, <literal>SQL</>, or <literal>German</>)
        and the input/output specification for year/month/day ordering
        (<literal>DMY</>, <literal>MDY</>, or <literal>YMD</>). These
        can be set separately or together. The keywords <literal>Euro</>
        and <literal>European</> are synonyms for <literal>DMY</>; the
        keywords <literal>US</>, <literal>NonEuro</>, and
        <literal>NonEuropean</> are synonyms for <literal>MDY</>. See
        <xref linkend="datatype-datetime"> for more information. The
        default is <literal>ISO, MDY</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-timezone" xreflabel="timezone">
      <term><varname>timezone</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>timezone</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone</></>
      <listitem>
       <para>
        Sets the time zone for displaying and interpreting time
        stamps.  The default is 'unknown', which means to use whatever 
        the system environment specifies as the time zone.  See <xref
        linkend="datatype-datetime"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-australian-timezones" xreflabel="australian_timezones">
      <term><varname>australian_timezones</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>australian_timezones</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone</><secondary>Australian</></>
      <listitem>
       <para>
        If set to true, <literal>ACST</literal>,
        <literal>CST</literal>, <literal>EST</literal>, and
        <literal>SAT</literal> are interpreted as Australian time
        zones rather than as North/South American time zones and
        Saturday. The default is false.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-extra-float-digits" xreflabel="extra_float_digits">
      <indexterm>
       <primary>significant digits</primary>
      </indexterm>
      <indexterm>
       <primary>floating-point</primary>
       <secondary>display</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>extra_float_digits</> configuration parameter</primary>
      </indexterm>

      <term><varname>extra_float_digits</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This parameter adjusts the number of digits displayed for
        floating-point values, including <type>float4</>, <type>float8</>,
        and geometric data types.  The parameter value is added to the
        standard number of digits (<literal>FLT_DIG</> or <literal>DBL_DIG</>
        as appropriate).  The value can be set as high as 2, to include
        partially-significant digits; this is especially useful for dumping
        float data that needs to be restored exactly.  Or it can be set
        negative to suppress unwanted digits.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-client-encoding" xreflabel="client_encoding">
      <term><varname>client_encoding</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>client_encoding</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</></>
      <listitem>
       <para>
        Sets the client-side encoding (character set).
        The default is to use the database encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-messages" xreflabel="lc_messages">
      <term><varname>lc_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the language in which messages are displayed.  Acceptable
        values are system-dependent; see <xref linkend="locale"> for
        more information.  If this variable is set to the empty string
        (which is the default) then the value is inherited from the
        execution environment of the server in a system-dependent way.
       </para>

       <para>
        On some systems, this locale category does not exist.  Setting
        this variable will still work, but there will be no effect.
        Also, there is a chance that no translated messages for the
        desired language exist.  In that case you will continue to see
        the English messages.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
      <term><varname>lc_monetary</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_monetary</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting monetary amounts, for
        example with the <function>to_char</function> family of
        functions.  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-numeric" xreflabel="lc_numeric">
      <term><varname>lc_numeric</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_numeric</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting numbers, for example
        with the <function>to_char</function> family of
        functions. Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-time" xreflabel="lc_time">
      <term><varname>lc_time</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_time</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting date and time values.
        (Currently, this setting does nothing, but it may in the
        future.)  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
     <sect3 id="runtime-config-client-other">
     <title>Other Defaults</title>

     <variablelist>

     <varlistentry id="guc-explain-pretty-print" xreflabel="explain_pretty_print">
      <term><varname>explain_pretty_print</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>explain_pretty_print</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Determines whether <command>EXPLAIN VERBOSE</> uses the
        indented or non-indented format for displaying detailed
        query-tree dumps. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
      <term><varname>dynamic_library_path</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>dynamic_library_path</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>dynamic loading</></>
      <listitem>
       <para>
        If a dynamically loadable module needs to be opened and the
        file name specified in the <command>CREATE FUNCTION</command> or
        <command>LOAD</command> command
        does not have a directory component (i.e. the
        name does not contain a slash), the system will search this
        path for the required file.
       </para>

       <para>
        The value for <varname>dynamic_library_path</varname> has to be a
        list of absolute directory paths separated by colons (or semi-colons
        on Windows).  If a list element starts
        with the special string <literal>$libdir</literal>, the
        compiled-in <productname>PostgreSQL</productname> package
        library directory is substituted for <literal>$libdir</literal>. This
        is where the modules provided by the standard
        <productname>PostgreSQL</productname> distribution are installed.
        (Use <literal>pg_config --pkglibdir</literal> to find out the name of
        this directory.) For example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
        or, in a Windows environment:
<programlisting>
dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</programlisting>
       </para>

       <para>
        The default value for this parameter is
        <literal>'$libdir'</literal>. If the value is set to an empty
        string, the automatic path search is turned off.
       </para>

       <para>
        This parameter can be changed at run time by superusers, but a
        setting done that way will only persist until the end of the
        client connection, so this method should be reserved for
        development purposes. The recommended way to set this parameter
        is in the <filename>postgresql.conf</filename> configuration
        file.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-locks">
    <title>Lock Management</title>

     <variablelist>

     <varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
      <indexterm>
       <primary>deadlock</primary>
       <secondary>timeout during</secondary>
      </indexterm>
      <indexterm>
       <primary>timeout</primary>
       <secondary>deadlock</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>deadlock_timeout</> configuration parameter</primary>
      </indexterm>

      <term><varname>deadlock_timeout</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This is the amount of time, in milliseconds, to wait on a lock
        before checking to see if there is a deadlock condition. The
        check for deadlock is relatively slow, so the server doesn't run
        it every time it waits for a lock. We (optimistically?) assume
        that deadlocks are not common in production applications and
        just wait on the lock for a while before starting the check for a
        deadlock. Increasing this value reduces the amount of time
        wasted in needless deadlock checks, but slows down reporting of
        real deadlock errors. The default is 1000 (i.e., one second),
        which is probably about the smallest value you would want in
        practice. On a heavily loaded server you might want to raise it.
        Ideally the setting should exceed your typical transaction time,
        so as to improve the odds that a lock will be released before
        the waiter decides to check for deadlock.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-locks-per-transaction" xreflabel="max_locks_per_transaction">
      <term><varname>max_locks_per_transaction</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_locks_per_transaction</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The shared lock table is sized on the assumption that at most
        <varname>max_locks_per_transaction</varname> *
        <varname>max_connections</varname> distinct objects will need to
        be locked at any one time. (Thus, this parameter's name may be
        confusing: it is not a hard limit on the number of locks taken
        by any one transaction, but rather a maximum average value.)
        The default, 64, has historically
        proven sufficient, but you might need to raise this value if you
        have clients that touch many different tables in a single
        transaction. This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
   </sect2>

   <sect2 id="runtime-config-compatible">
    <title>Version and Platform Compatibility</title>

    <sect3 id="runtime-config-compatible-version">
     <title>Previous PostgreSQL Versions</title>
     <variablelist>

     <varlistentry id="guc-add-missing-from" xreflabel="add_missing_from">
      <term><varname>add_missing_from</varname> (<type>boolean</type>)</term>
      <indexterm><primary>FROM</><secondary>missing</></>
      <indexterm>
       <primary><varname>add_missing_from</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <literal>true</>, tables that are referenced by a query will be
        automatically added to the <literal>FROM</> clause if not already
        present.  The default is <literal>true</> for compatibility with
        previous releases of <productname>PostgreSQL</>.  However, this
        behavior is not SQL-standard, and many people dislike it because it
        can mask mistakes (such as referencing a table where you should have
        referenced its alias).  Set to <literal>false</> for the SQL-standard
        behavior of rejecting references to tables that are not listed in
        <literal>FROM</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-backslash-quote" xreflabel="backslash_quote">
      <term><varname>backslash_quote</varname> (<type>string</type>)</term>
      <indexterm><primary>strings</><secondary>backslash quotes</></>
      <indexterm>
       <primary><varname>backslash_quote</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether a quote mark can be represented by
        <literal>\'</> in a string literal.  The preferred, SQL-standard way
        to represent a quote mark is by doubling it (<literal>''</>) but
        <productname>PostgreSQL</> has historically also accepted
        <literal>\'</>. However, use of <literal>\'</> creates security risks
        because in some client character set encodings, there are multibyte
        characters in which the last byte is numerically equivalent to ASCII
        <literal>\</>.  If client-side code does escaping incorrectly then a
        SQL-injection attack is possible.  This risk can be prevented by
        making the server reject queries in which a quote mark appears to be
        escaped by a backslash.
        The allowed values of <varname>backslash_quote</> are
        <literal>on</> (allow <literal>\'</> always),
        <literal>off</> (reject always), and
        <literal>safe_encoding</> (allow only if client encoding does not
        allow ASCII <literal>\</> within a multibyte character).
        <literal>safe_encoding</> is the default setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-with-oids" xreflabel="default_with_oids">
      <term><varname>default_with_oids</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>default_with_oids</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether <command>CREATE TABLE</command> and
        <command>CREATE TABLE AS</command> include an OID column in
        newly-created tables, if neither <literal>WITH OIDS</literal>
        nor <literal>WITHOUT OIDS</literal> is specified. It also
        determines whether OIDs will be included in tables created by
        <command>SELECT INTO</command>. In
        <productname>PostgreSQL</productname> &version;
        <varname>default_with_oids</varname> defaults to true. This is
        also the behavior of previous versions of
        <productname>PostgreSQL</productname>. However, assuming that
        tables will contain OIDs by default is not encouraged. This
        option will probably default to false in a future release of
        <productname>PostgreSQL</productname>.
       </para>

       <para>
        To ease compatibility with applications that make use of OIDs,
        this option should left enabled. To ease compatibility with
        future versions of <productname>PostgreSQL</productname>, this
        option should be disabled, and applications that require OIDs
        on certain tables should explicitly specify <literal>WITH
        OIDS</literal> when those tables are created.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-regex-flavor" xreflabel="regex_flavor">
      <term><varname>regex_flavor</varname> (<type>string</type>)</term>
      <indexterm><primary>regular expressions</></>
      <indexterm>
       <primary><varname>regex_flavor</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The regular expression <quote>flavor</> can be set to
        <literal>advanced</>, <literal>extended</>, or <literal>basic</>.
        The default is <literal>advanced</>.  The <literal>extended</>
        setting may be useful for exact backwards compatibility with
        pre-7.4 releases of <productname>PostgreSQL</>.  See
        <xref linkend="posix-syntax-details"> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-sql-inheritance" xreflabel="sql_inheritance">
      <term><varname>sql_inheritance</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>sql_inheritance</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>inheritance</></>
      <listitem>
       <para>
        This controls the inheritance semantics, in particular whether
        subtables are included by various commands by default. They were
        not included in versions prior to 7.1. If you need the old
        behavior you can set this variable to off, but in the long run
        you are encouraged to change your applications to use the
        <literal>ONLY</literal> key word to exclude subtables. See
        <xref linkend="ddl-inherit"> for more information about inheritance.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
    <sect3 id="runtime-config-compatible-clients">
     <title>Platform and Client Compatibility</title>
     <variablelist>

     <varlistentry id="guc-transform-null-equals" xreflabel="transform_null_equals">
      <term><varname>transform_null_equals</varname> (<type>boolean</type>)</term>
      <indexterm><primary>IS NULL</></>
      <indexterm>
       <primary><varname>transform_null_equals</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When turned on, expressions of the form
        <literal><replaceable>expr</> = NULL</literal> (or <literal>NULL
        = <replaceable>expr</></literal>) are treated as
        <literal><replaceable>expr</> IS NULL</literal>, that is, they
        return true if <replaceable>expr</> evaluates to the null value,
        and false otherwise. The correct SQL-spec-compliant behavior of
        <literal><replaceable>expr</> = NULL</literal> is to always
        return null (unknown). Therefore this option defaults to off.
       </para>

       <para>
        However, filtered forms in <productname>Microsoft
        Access</productname> generate queries that appear to use
        <literal><replaceable>expr</> = NULL</literal> to test for
        null values, so if you use that interface to access the database you
        might want to turn this option on.  Since expressions of the
        form <literal><replaceable>expr</> = NULL</literal> always
        return the null value (using the correct interpretation) they are not
        very useful and do not appear often in normal applications, so
        this option does little harm in practice.  But new users are
        frequently confused about the semantics of expressions
        involving null values, so this option is not on by default.
       </para>

       <para>
        Note that this option only affects the exact form <literal>= NULL</>,
        not other comparison operators or other expressions
        that are computationally equivalent to some expression
        involving the equals operator (such as <literal>IN</literal>).
        Thus, this option is not a general fix for bad programming.
       </para>

       <para>
        Refer to <xref linkend="functions-comparison"> for related information.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-preset">
    <title>Preset Options</title>

    <para>
     The following <quote>parameters</> are read-only, and are determined
     when <productname>PostgreSQL</productname> is compiled or when it is
     installed. As such, they have been excluded from the sample
     <filename>postgresql.conf</> file.  These options report
     various aspects of <productname>PostgreSQL</productname> behavior
     that may be of interest to certain applications, particularly
     administrative front-ends.
    </para>

    <variablelist>

     <varlistentry id="guc-block-size" xreflabel="block_size">
      <term><varname>block_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>block_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the size of a disk block.  It is determined by the value
        of <literal>BLCKSZ</> when building the server. The default
        value is 8192 bytes.  The meaning of some configuration
        variables (such as <xref linkend="guc-shared-buffers">) is
        influenced by <varname>block_size</varname>. See <xref
        linkend="runtime-config-resource"> for information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
      <term><varname>integer_datetimes</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>integer_datetimes</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows whether <productname>PostgreSQL</productname> was built
        with support for 64-bit-integer dates and times.  It is set by
        configuring with <literal>--enable-integer-datetimes</literal>
        when building <productname>PostgreSQL</productname>.  The
        default value is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-collate" xreflabel="lc_collate">
      <term><varname>lc_collate</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_collate</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the locale in which sorting of textual data is done.
        See <xref linkend="locale"> for more information.
        The value is determined when the database cluster is initialized.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
      <term><varname>lc_ctype</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_ctype</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the locale that determines character classifications.
        See <xref linkend="locale"> for more information.
        The value is determined when the database cluster is initialized.
        Ordinarily this will be the same as <varname>lc_collate</varname>,
        but for special applications it might be set differently.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-function-args" xreflabel="max_function_args">
      <term><varname>max_function_args</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_function_args</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the maximum number of function arguments. It is determined by
        the value of <literal>FUNC_MAX_ARGS</> when building the server. The
        default value is 32.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-identifier-length" xreflabel="max_identifier_length">
      <term><varname>max_identifier_length</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_identifier_length</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the maximum identifier length. It is determined as one
        less than the value of <literal>NAMEDATALEN</> when building
        the server. The default value of <literal>NAMEDATALEN</> is
        64; therefore the default
        <varname>max_identifier_length</varname> is 63.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
      <term><varname>max_index_keys</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_index_keys</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the maximum number of index keys. It is determined by
        the value of <literal>INDEX_MAX_KEYS</> when building the server. The
        default value is 32.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-encoding" xreflabel="server_encoding">
      <term><varname>server_encoding</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>server_encoding</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</></>
      <listitem>
       <para>
        Shows the database encoding (character set).
        It is determined when the database is created.  Ordinarily,
        clients need only be concerned with the value of <xref
        linkend="guc-client-encoding">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-version" xreflabel="server_version">
      <term><varname>server_version</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>server_version</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Shows the version number of the server. It is determined by the
        value of <literal>PG_VERSION</> when building the server.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect2>

   <sect2 id="runtime-config-custom">
    <title>Customized Options</title>

    <para>
     This feature was designed to allow options not normally known to
     <productname>PostgreSQL</productname> to be added by add-on modules
     (such as procedural languages).  This allows add-on modules to be
     configured in the standard ways.
    </para>

    <variablelist>

     <varlistentry id="guc-custom-variable-classes" xreflabel="custom_variable_classes">
      <term><varname>custom_variable_classes</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>custom_variable_classes</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This variable specifies one or several class names to be used for
        custom variables, in the form of a comma-separated list. A custom
        variable is a variable not normally known
        to <productname>PostgreSQL</productname> proper but used by some
        add-on module.  Such variables must have names consisting of a class
        name, a dot, and a variable name.  <varname>custom_variable_classes</>
        specifies all the class names in use in a particular installation.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>

      </listitem>
     </varlistentry>
    </variablelist>

    <para>
     The difficulty with setting custom variables in
     <filename>postgresql.conf</> is that the file must be read before add-on
     modules have been loaded, and so custom variables would ordinarily be
     rejected as unknown.  When <varname>custom_variable_classes</> is set,
     the server will accept definitions of arbitrary variables within each
     specified class.  These variables will be treated as placeholders and
     will have no function until the module that defines them is loaded. When a
     module for a specific class is loaded, it will add the proper variable
     definitions for its class name, convert any placeholder
     values according to those definitions, and issue warnings for any
     placeholders of its class that remain (which presumably would be
     misspelled configuration variables).
    </para>

    <para>
     Here is an example of what <filename>postgresql.conf</> might contain
     when using custom variables:

<programlisting>
custom_variable_classes = 'plr,pljava'
plr.path = '/usr/lib/R'
pljava.foo = 1
plruby.bar = true        # generates error, unknown class name
</programlisting>
    </para>
   </sect2>

   <sect2 id="runtime-config-developer">
    <title>Developer Options</title>

    <para>
     The following options are intended for work on the
     <productname>PostgreSQL</productname> source, and in some cases
     to assist with recovery of severely damaged databases.  There
     should be no reason to use them in a production database setup.
     As such, they have been excluded from the sample
     <filename>postgresql.conf</> file.  Note that many of these
     options require special source compilation flags to work at all.
    </para>

    <variablelist>
     <varlistentry id="guc-debug-assertions" xreflabel="debug_assertions">
      <term><varname>debug_assertions</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>debug_assertions</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Turns on various assertion checks. This is a debugging aid. If
        you are experiencing strange problems or crashes you might want
        to turn this on, as it might expose programming mistakes. To use
        this option, the macro <symbol>USE_ASSERT_CHECKING</symbol>
        must be defined when <productname>PostgreSQL</productname> is
        built (accomplished by the <command>configure</command> option
        <option>--enable-cassert</option>). Note that
        <varname>debug_assertions</varname> defaults to on if
        <productname>PostgreSQL</productname> has been built with
        assertions enabled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-debug-shared-buffers" xreflabel="debug_shared_buffers">
      <term><varname>debug_shared_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>debug_shared_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Number of seconds between buffer freelist reports.
        If set greater than zero, emit freelist statistics to the log every so many
        seconds.  Zero (the default) disables reporting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-pre-auth-delay" xreflabel="pre_auth_delay">
      <term><varname>pre_auth_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>pre_auth_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs just after a new
        server process is forked, before it conducts the authentication
        process.  This is intended to give an opportunity to attach to the
        server process with a debugger to trace down misbehavior in
        authentication.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-notify" xreflabel="trace_notify">
      <term><varname>trace_notify</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>trace_notify</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Generates a great amount of debugging output for the
        <command>LISTEN</command> and <command>NOTIFY</command>
        commands.  <xref linkend="guc-client-min-messages"> or
        <xref linkend="guc-log-min-messages"> must be
        <literal>DEBUG1</literal> or lower to send this output to the
        client or server log, respectively.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_locks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lwlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_userlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_oidmin</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_table</varname> (<type>boolean</type>)</term>
      <term><varname>debug_deadlocks</varname> (<type>boolean</type>)</term>
      <term><varname>log_btree_build_stats</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Various other code tracing and debugging options.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-debug" xreflabel="wal_debug">
      <term><varname>wal_debug</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>wal_debug</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If true, emit WAL-related debugging output. This option is
        only available if the <symbol>WAL_DEBUG</symbol> macro was
        defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry id="guc-zero-damaged-pages" xreflabel="zero_damaged_pages">
      <term><varname>zero_damaged_pages</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>zero_damaged_pages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Detection of a damaged page header normally causes
        <productname>PostgreSQL</> to report an error, aborting the current
        command.  Setting <varname>zero_damaged_pages</> to true causes
        the system to instead report a warning, zero out the damaged page,
        and continue processing.  This behavior <emphasis>will destroy data</>,
        namely all the rows on the damaged page.  But it allows you to get
        past the error and retrieve rows from any undamaged pages that may
        be present in the table.  So it is useful for recovering data if
        corruption has occurred due to hardware or software error.  You should
        generally not set this true until you have given up hope of recovering
        data from the damaged page(s) of a table.  The
        default setting is off, and it can only be changed by a superuser.
       </para>
      </listitem>
     </varlistentry>
   </variablelist>
  </sect2>
  <sect2 id="runtime-config-short">
   <title>Short Options</title>

   <para>
    For convenience there are also single letter command-line option switches
    available for some parameters. They are described in <xref
    linkend="runtime-config-short-table">.
   </para>

    <table id="runtime-config-short-table">
     <title>Short option key</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Short option</entry>
        <entry>Equivalent</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><option>-B <replaceable>x</replaceable></option></entry>
        <entry><literal>shared_buffers = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-d <replaceable>x</replaceable></option></entry>
        <entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-F</option></entry>
        <entry><literal>fsync = off</></entry>
       </row>
       <row>
        <entry><option>-h <replaceable>x</replaceable></option></entry>
        <entry><literal>listen_addresses = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-i</option></entry>
        <entry><literal>listen_addresses = '*'</></entry>
       </row>
       <row>
        <entry><option>-k <replaceable>x</replaceable></option></entry>
        <entry><literal>unix_socket_directory = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-l</option></entry>
        <entry><literal>ssl = on</></entry>
       </row>
       <row>
        <entry><option>-N <replaceable>x</replaceable></option></entry>
        <entry><literal>max_connections = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-p <replaceable>x</replaceable></option></entry>
        <entry><literal>port = <replaceable>x</replaceable></></entry>
       </row>

       <row>
        <entry>
          <option>-fi</option>, <option>-fh</option>,
          <option>-fm</option>, <option>-fn</option>,
          <option>-fs</option>, <option>-ft</option><footnote
          id="fn.runtime-config-short">
           <para>
            For historical reasons, these options must be passed to
            the individual server process via the <option>-o</option>
            <command>postmaster</command> option, for example,
<screen>
$ <userinput>postmaster -o '-S 1024 -s'</userinput>
</screen>
            or via <envar>PGOPTIONS</envar> from the client side, as
            explained above.
           </para>
          </footnote>
         </entry>
         <entry>
          <literal>enable_indexscan = off</>,
          <literal>enable_hashjoin = off</>,
          <literal>enable_mergejoin = off</>,
          <literal>enable_nestloop = off</>,
          <literal>enable_seqscan = off</>,
          <literal>enable_tidscan = off</>
         </entry>
       </row>

       <row>
        <entry><option>-s</option><footnoteref linkend="fn.runtime-config-short"></entry>
        <entry><literal>log_statement_stats = on</></entry>
       </row>

       <row>
        <entry><option>-S <replaceable>x</replaceable></option><footnoteref linkend="fn.runtime-config-short">
        </entry>
        <entry><literal>work_mem = <replaceable>x</replaceable></></entry>
       </row>

       <row>
        <entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option><footnoteref linkend="fn.runtime-config-short"></entry>
        <entry><literal>log_parser_stats = on</>,
        <literal>log_planner_stats = on</>, 
        <literal>log_executor_stats = on</></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect2>
 </sect1>


 <sect1 id="kernel-resources">
  <title>Managing Kernel Resources</title>

  <para>
   A large <productname>PostgreSQL</> installation can quickly exhaust
   various operating system resource limits. (On some systems, the
   factory defaults are so low that you don't even need a really
   <quote>large</> installation.) If you have encountered this kind of
   problem, keep reading.
  </para>

  <sect2 id="sysvipc">
   <title>Shared Memory and Semaphores</title>

   <indexterm zone="sysvipc">
    <primary>shared memory</primary>
   </indexterm>

   <indexterm zone="sysvipc">
    <primary>semaphores</primary>
   </indexterm>

   <para>
    Shared memory and semaphores are collectively referred to as
    <quote><systemitem class="osname">System V</>
    <acronym>IPC</></quote> (together with message queues, which are not
    relevant for <productname>PostgreSQL</>). Almost all modern
    operating systems provide these features, but not all of them have
    them turned on or sufficiently sized by default, especially systems
    with BSD heritage. (For the <systemitem class="osname">QNX</> and
    <systemitem class="osname">BeOS</> ports, <productname>PostgreSQL</>
    provides its own replacement implementation of these facilities.)
   </para>

   <para>
    The complete lack of these facilities is usually manifested by an
    <errorname>Illegal system call</> error upon server start. In
    that case there's nothing left to do but to reconfigure your
    kernel.  <productname>PostgreSQL</> won't work without them.
   </para>

   <para>
    When <productname>PostgreSQL</> exceeds one of the various hard
    <acronym>IPC</> limits, the server will refuse to start and
    should leave an instructive error message describing the problem
    encountered and what to do about it. (See also <xref
    linkend="postmaster-start-failures">.) The relevant kernel
    parameters are named consistently across different systems; <xref
    linkend="sysvipc-parameters"> gives an overview. The methods to set
    them, however, vary. Suggestions for some platforms are given below.
    Be warned that it is often necessary to reboot your machine, and
    possibly even recompile the kernel, to change these settings.
   </para>


   <table id="sysvipc-parameters">
    <title><systemitem class="osname">System V</> <acronym>IPC</> parameters</>

    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</>
       <entry>Description</>
       <entry>Reasonable values</>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><varname>SHMMAX</></>
       <entry>Maximum size of shared memory segment (bytes)</>
       <entry>250 kB + 8.2 kB * <xref linkend="guc-shared-buffers"> +
       14.2 kB * <xref linkend="guc-max-connections"> up to infinity</entry>
      </row>

      <row>
       <entry><varname>SHMMIN</></>
       <entry>Minimum size of shared memory segment (bytes)</>
       <entry>1</>
      </row>

      <row>
       <entry><varname>SHMALL</></>
       <entry>Total amount of shared memory available (bytes or pages)</>
       <entry>if bytes, same as <varname>SHMMAX</varname>; if pages, <literal>ceil(SHMMAX/PAGE_SIZE)</literal></>
      </row>

      <row>
       <entry><varname>SHMSEG</></>
       <entry>Maximum number of shared memory segments per process</>
       <entry>only 1 segment is needed, but the default is much higher</>
      </row>

       <row>
        <entry><varname>SHMMNI</></>
        <entry>Maximum number of shared memory segments system-wide</>
        <entry>like <varname>SHMSEG</> plus room for other applications</>
       </row>

       <row>
        <entry><varname>SEMMNI</></>
        <entry>Maximum number of semaphore identifiers (i.e., sets)</>
        <entry>at least <literal>ceil(max_connections / 16)</literal></>
       </row>

       <row>
        <entry><varname>SEMMNS</></>
        <entry>Maximum number of semaphores system-wide</>
        <entry><literal>ceil(max_connections / 16) * 17</literal> plus room for other applications</>
       </row>

       <row>
        <entry><varname>SEMMSL</></>
        <entry>Maximum number of semaphores per set</>
        <entry>at least 17</>
       </row>

       <row>
        <entry><varname>SEMMAP</></>
        <entry>Number of entries in semaphore map</>
        <entry>see text</>
       </row>

       <row>
        <entry><varname>SEMVMX</></>
        <entry>Maximum value of semaphore</>
        <entry>at least 1000 (The default is often 32767, don't change unless forced to)</>
       </row>

     </tbody>
    </tgroup>
   </table>


   <para>
    <indexterm><primary>SHMMAX</primary></indexterm> The most important
    shared memory parameter is <varname>SHMMAX</>, the maximum size, in
    bytes, of a shared memory segment. If you get an error message from
    <function>shmget</> like <errorname>Invalid argument</>, it is
    likely that this limit has been exceeded. The size of the required
    shared memory segment varies both with the number of requested
    buffers (<option>-B</> option) and the number of allowed connections
    (<option>-N</> option), although the former is the most significant.
    (You can, as a temporary solution, lower these settings to eliminate
    the failure.) As a rough approximation, you can estimate the
    required segment size as suggested in <xref
    linkend="sysvipc-parameters">.  Any error message you might get will
    contain the size of the failed allocation request.
   </para>

   <para>
    Some systems also have a limit on the total amount of shared memory in
    the system (<varname>SHMALL</>).  Make sure this is large enough
    for <productname>PostgreSQL</> plus any other applications that
    are using shared memory segments.  (Caution: <varname>SHMALL</>
    is measured in pages rather than bytes on many systems.)
   </para>

   <para>
    Less likely to cause problems is the minimum size for shared
    memory segments (<varname>SHMMIN</>), which should be at most
    approximately 256 kB for <productname>PostgreSQL</> (it is
    usually just 1). The maximum number of segments system-wide
    (<varname>SHMMNI</>) or per-process (<varname>SHMSEG</>) are unlikely
    to cause a problem unless your system has them set to zero.
   </para>

   <para>
    <productname>PostgreSQL</> uses one semaphore per allowed connection
    (<option>-N</> option), in sets of 16.  Each such set will also
    contain a 17th semaphore which contains a <quote>magic
    number</quote>, to detect collision with semaphore sets used by
    other applications. The maximum number of semaphores in the system
    is set by <varname>SEMMNS</>, which consequently must be at least
    as high as <varname>max_connections</> plus one extra for each 16
    allowed connections (see the formula in <xref
    linkend="sysvipc-parameters">).  The parameter <varname>SEMMNI</>
    determines the limit on the number of semaphore sets that can
    exist on the system at one time.  Hence this parameter must be at
    least <literal>ceil(max_connections / 16)</>. Lowering the number
    of allowed connections is a temporary workaround for failures,
    which are usually confusingly worded <errorname>No space
    left on device</>, from the function <function>semget</>.
   </para>

   <para>
    In some cases it might also be necessary to increase
    <varname>SEMMAP</> to be at least on the order of
    <varname>SEMMNS</>. This parameter defines the size of the semaphore
    resource map, in which each contiguous block of available semaphores
    needs an entry. When a semaphore set is freed it is either added to
    an existing entry that is adjacent to the freed block or it is
    registered under a new map entry. If the map is full, the freed
    semaphores get lost (until reboot). Fragmentation of the semaphore
    space could over time lead to fewer available semaphores than there
    should be.
   </para>

   <para>
    The <varname>SEMMSL</> parameter, which determines how many
    semaphores can be in a set, must be at least 17 for
    <productname>PostgreSQL</>.
   </para>

   <para>
    Various other settings related to <quote>semaphore undo</>, such as
    <varname>SEMMNU</> and <varname>SEMUME</>, are not of concern
    for <productname>PostgreSQL</>.
   </para>

    <variablelist>

     <varlistentry>
      <term><systemitem class="osname">BSD/OS</></term>
      <indexterm><primary>BSD/OS</><secondary>IPC configuration</></>
      <listitem>
       <formalpara>
        <title>Shared Memory</>
        <para>
         By default, only 4 MB of shared memory is supported. Keep in
         mind that shared memory is not pageable; it is locked in RAM.
         To increase the amount of shared memory supported by your
         system, add something like the following to your kernel configuration
         file:
<programlisting>
options "SHMALL=8192"
options "SHMMAX=\(SHMALL*PAGE_SIZE\)"
</programlisting>
         <varname>SHMALL</> is measured in 4KB pages, so a value of
         1024 represents 4 MB of shared memory. Therefore the above increases
         the maximum shared memory area to 32 MB.
         For those running 4.3 or later, you will probably also need to increase
         <varname>KERNEL_VIRTUAL_MB</> above the default <literal>248</>.
         Once all changes have been made, recompile the kernel, and reboot.
        </para>
       </formalpara>

       <para>
        For those running 4.0 and earlier releases, use <command>bpatch</>
        to find the <varname>sysptsize</> value in the current
        kernel. This is computed dynamically at boot time.
<screen>
$ <userinput>bpatch -r sysptsize</>
<computeroutput>0x9 = 9</>
</screen>
        Next, add <varname>SYSPTSIZE</> as a hard-coded value in the
        kernel configuration file. Increase the value you found using
        <command>bpatch</>. Add 1 for every additional 4 MB of
        shared memory you desire.
<programlisting>
options "SYSPTSIZE=16"
</programlisting>
        <varname>sysptsize</> cannot be changed by <command>sysctl</command>.
       </para>

       <formalpara>
        <title>Semaphores</>
        <para>
         You will probably want to increase the number of semaphores
         as well; the default system total of 60 will only allow about
         50 <productname>PostgreSQL</productname> connections.  Set the
         values you want in your kernel configuration file, e.g.:
<programlisting>
options "SEMMNI=40"
options "SEMMNS=240"
</programlisting>
        </para>
       </formalpara>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">FreeBSD</></term>
      <term><systemitem class="osname">NetBSD</></term>
      <term><systemitem class="osname">OpenBSD</></term>
      <indexterm><primary>FreeBSD</><secondary>IPC configuration</></>
      <indexterm><primary>NetBSD</><secondary>IPC configuration</></>
      <indexterm><primary>OpenBSD</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The options <varname>SYSVSHM</> and <varname>SYSVSEM</> need
        to be enabled when the kernel is compiled. (They are by
        default.) The maximum size of shared memory is determined by
        the option <varname>SHMMAXPGS</> (in pages). The following
        shows an example of how to set the various parameters:
<programlisting>
options         SYSVSHM
options         SHMMAXPGS=4096
options         SHMSEG=256

options         SYSVSEM
options         SEMMNI=256
options         SEMMNS=512
options         SEMMNU=256
options         SEMMAP=256
</programlisting>
        (On <systemitem class="osname">OpenBSD</> the key word is actually
        <literal>option</literal> singular.)
       </para>
       <para>
        You might also want to configure your kernel to lock shared
        memory into RAM and prevent it from being paged out to swap.
        Use the <command>sysctl</> setting
        <literal>kern.ipc.shm_use_phys</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">HP-UX</></term>
      <indexterm><primary>HP-UX</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The default settings tend to suffice for normal installations.
        On <productname>HP-UX</> 10, the factory default for
        <varname>SEMMNS</> is 128, which might be too low for larger
        database sites.
       </para>
       <para>
        <acronym>IPC</> parameters can be set in the <application>System
        Administration Manager</> (<acronym>SAM</>) under
        <menuchoice><guimenu>Kernel
        Configuration</><guimenuitem>Configurable Parameters</></>. Hit
        <guibutton>Create A New Kernel</> when you're done.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">Linux</></term>
      <indexterm><primary>Linux</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The default shared memory limit (both
        <varname>SHMMAX</varname> and <varname>SHMALL</varname>) is 32
        MB in 2.2 kernels, but it can be changed in the
        <filename>proc</filename> file system (without reboot).  For
        example, to allow 128 MB:
<screen>
<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmall</userinput>
<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmmax</userinput>
</screen>
        You could put these commands into a script run at boot-time.
       </para>

       <para>
        Alternatively, you can use <command>sysctl</command>, if
        available, to control these parameters.  Look for a file
        called <filename>/etc/sysctl.conf</filename> and add lines
        like the following to it:
<programlisting>
kernel.shmall = 134217728
kernel.shmmax = 134217728
</programlisting>
        This file is usually processed at boot time, but
        <command>sysctl</command> can also be called
        explicitly later.
       </para>

       <para>
        Other parameters are sufficiently sized for any application. If
        you want to see for yourself look in
        <filename>/usr/src/linux/include/asm-<replaceable>xxx</>/shmparam.h</>
        and <filename>/usr/src/linux/include/linux/sem.h</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">MacOS X</></term>
      <indexterm><primary>MacOS X</><secondary>IPC configuration</></>
      <listitem>
       <para>
        In OS X 10.2 and earlier, edit the file
        <filename>/System/Library/StartupItems/SystemTuning/SystemTuning</>
        and change the values in the following commands:
<programlisting>
sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall
</programlisting>
        In OS X 10.3, these commands have been moved to <filename>/etc/rc</>
        and must be edited there.  You'll need to reboot to make changes
        take effect.  Note that <filename>/etc/rc</> is usually
        overwritten by OS X updates (such as 10.3.6 to 10.3.7) so you
        should expect to have to redo your editing after each update.
       </para>
       <para>
        <varname>SHMALL</> is measured in 4KB pages on this platform.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">SCO OpenServer</></term>
      <indexterm><primary>SCO OpenServer</><secondary>IPC configuration</></>
      <listitem>
       <para>
        In the default configuration, only 512 kB of shared memory per
        segment is allowed, which is about enough for <option>-B 24 -N
        12</>. To increase the setting, first change to the directory
        <filename>/etc/conf/cf.d</>. To display the current value of
        <varname>SHMMAX</>, run
<programlisting>
./configure -y SHMMAX
</programlisting>
        To set a new value for <varname>SHMMAX</>, run
<programlisting>
./configure SHMMAX=<replaceable>value</>
</programlisting>
        where <replaceable>value</> is the new value you want to use
        (in bytes). After setting <varname>SHMMAX</>, rebuild the kernel:
<programlisting>
./link_unix
</programlisting>
        and reboot.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><systemitem class="osname">AIX</></term>
      <indexterm><primary>AIX</><secondary>IPC configuration</></>
      <listitem>
       <para>
        At least as of version 5.1, it should not be necessary to do
        any special configuration for such parameters as
        <varname>SHMMAX</varname>, as it appears this is configured to
        allow all memory to be used as shared memory.  That is the
        sort of configuration commonly used for other databases such
        as <application>DB/2</application>.</para>

       <para> It may, however, be necessary to modify the global
       <command>ulimit</command> information in
       <filename>/etc/security/limits</filename>, as the default hard
       limits for file sizes (<varname>fsize</varname>) and numbers of
       files (<varname>nofiles</varname>) may be too low.
       </para>
      </listitem>
     </varlistentry>      

     <varlistentry>
      <term><systemitem class="osname">Solaris</></term>
      <indexterm><primary>Solaris</><secondary>IPC configuration</></>
      <listitem>
       <para>
        At least in version 2.6, the default maximum size of a shared
        memory segments is too low for <productname>PostgreSQL</>. The
        relevant settings can be changed in <filename>/etc/system</>,
        for example:
<programlisting>
set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256

set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32
</programlisting>
        You need to reboot for the changes to take effect.
       </para>

       <para>
        See also <ulink
        url="http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html"></>
        for information on shared memory under
        <productname>Solaris</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">UnixWare</></term>
      <indexterm><primary>UnixWare</><secondary>IPC configuration</></>
      <listitem>
       <para>
        On <productname>UnixWare</> 7, the maximum size for shared
        memory segments is 512 kB in the default configuration. This
        is enough for about <option>-B 24 -N 12</>. To display the
        current value of <varname>SHMMAX</>, run
<programlisting>
/etc/conf/bin/idtune -g SHMMAX
</programlisting>
        which displays the current, default, minimum, and maximum
        values. To set a new value for <varname>SHMMAX</>,
        run
<programlisting>
/etc/conf/bin/idtune SHMMAX <replaceable>value</>
</programlisting>
        where <replaceable>value</> is the new value you want to use
        (in bytes). After setting <varname>SHMMAX</>, rebuild the
        kernel:
<programlisting>
/etc/conf/bin/idbuild -B
</programlisting>
        and reboot.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

  </sect2>


  <sect2>
   <title>Resource Limits</title>

   <para>
    Unix-like operating systems enforce various kinds of resource limits
    that might interfere with the operation of your
    <productname>PostgreSQL</productname> server. Of particular
    importance are limits on the number of processes per user, the
    number of open files per process, and the amount of memory available
    to each process. Each of these have a <quote>hard</quote> and a
    <quote>soft</quote> limit. The soft limit is what actually counts
    but it can be changed by the user up to the hard limit. The hard
    limit can only be changed by the root user. The system call
    <function>setrlimit</function> is responsible for setting these
    parameters. The shell's built-in command <command>ulimit</command>
    (Bourne shells) or <command>limit</command> (<application>csh</>) is
    used to control the resource limits from the command line. On
    BSD-derived systems the file <filename>/etc/login.conf</filename>
    controls the various resource limits set during login. See the
    operating system documentation for details. The relevant
    parameters are <varname>maxproc</varname>,
    <varname>openfiles</varname>, and <varname>datasize</varname>. For
    example:
<programlisting>
default:\
...
        :datasize-cur=256M:\
        :maxproc-cur=256:\
        :openfiles-cur=256:\
...
</programlisting>
    (<literal>-cur</literal> is the soft limit.  Append
    <literal>-max</literal> to set the hard limit.)
   </para>

   <para>
    Kernels can also have system-wide limits on some resources.
    <itemizedlist>
     <listitem>
      <para>
      On <productname>Linux</productname>
      <filename>/proc/sys/fs/file-max</filename> determines the
      maximum number of open files that the kernel will support.  It can
      be changed by writing a different number into the file or by
      adding an assignment in <filename>/etc/sysctl.conf</filename>.
      The maximum limit of files per process is fixed at the time the
      kernel is compiled; see
      <filename>/usr/src/linux/Documentation/proc.txt</filename> for
      more information.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The <productname>PostgreSQL</productname> server uses one process
    per connection so you should provide for at least as many processes
    as allowed connections, in addition to what you need for the rest
    of your system.  This is usually not a problem but if you run
    several servers on one machine things might get tight.
   </para>

   <para>
    The factory default limit on open files is often set to
    <quote>socially friendly</quote> values that allow many users to
    coexist on a machine without using an inappropriate fraction of
    the system resources.  If you run many servers on a machine this
    is perhaps what you want, but on dedicated servers you may want to
    raise this limit.
   </para>

   <para>
    On the other side of the coin, some systems allow individual
    processes to open large numbers of files; if more than a few
    processes do so then the system-wide limit can easily be exceeded.
    If you find this happening, and you do not want to alter the
    system-wide limit, you can set <productname>PostgreSQL</>'s <xref
    linkend="guc-max-files-per-process"> configuration parameter to
    limit the consumption of open files.
   </para>
  </sect2>

  <sect2>
   <title>Linux Memory Overcommit</title>

   <para>
    In Linux 2.4 and later, the default virtual memory behavior is not
    optimal for <productname>PostgreSQL</productname>. Because of the
    way that the kernel implements memory overcommit, the kernel may
    terminate the <productname>PostgreSQL</productname> server (the
    <filename>postmaster</filename> process) if the memory demands of
    another process cause the system to run out of virtual memory.
   </para>

   <para>
    If this happens, you will see a kernel message that looks like
    this (consult your system documentation and configuration on where
    to look for such a message):
<programlisting>
Out of Memory: Killed process 12345 (postmaster). 
</programlisting>
    This indicates that the <filename>postmaster</filename> process
    has been terminated due to memory pressure.
    Although existing database connections will continue to function
    normally, no new connections will be accepted.  To recover,
    <productname>PostgreSQL</productname> will need to be restarted.
   </para>

   <para>
    One way to avoid this problem is to run
    <productname>PostgreSQL</productname>
    on a machine where you can be sure that other processes will not
    run the machine out of memory.
   </para>

   <para>
    On Linux 2.6 and later, a better solution is to modify the kernel's
    behavior so that it will not <quote>overcommit</> memory.  This is
    done by selecting strict overcommit mode via <command>sysctl</command>:
<programlisting>
sysctl -w vm.overcommit_memory=2
</programlisting>
    or placing an equivalent entry in <filename>/etc/sysctl.conf</>.
    You may also wish to modify the related setting 
    <literal>vm.overcommit_ratio</>.  For details see the kernel documentation
    file <filename>Documentation/vm/overcommit-accounting</>.
   </para>

   <para>
    Some vendors' Linux 2.4 kernels are reported to have early versions
    of the 2.6 overcommit <command>sysctl</command> parameter.  However, setting
    <literal>vm.overcommit_memory</> to 2
    on a kernel that does not have the relevant code will make
    things worse not better.  It is recommended that you inspect
    the actual kernel source code (see the function
    <function>vm_enough_memory</> in the file <filename>mm/mmap.c</>)
    to verify what is supported in your copy before you try this in a 2.4
    installation.  The presence of the <filename>overcommit-accounting</>
    documentation file should <emphasis>not</> be taken as evidence that the
    feature is there.  If in any doubt, consult a kernel expert or your
    kernel vendor.
   </para>
  </sect2>
 </sect1>


 <sect1 id="postmaster-shutdown">
  <title>Shutting Down the Server</title>

  <indexterm zone="postmaster-shutdown">
   <primary>shutdown</>
  </indexterm>

  <para>
   There are several ways to shut down the database server. You control
   the type of shutdown by sending different signals to the
   <command>postmaster</command> process.

   <variablelist>
    <varlistentry>
     <term><systemitem>SIGTERM</systemitem><indexterm><primary>SIGTERM</></></term>
     <listitem>
      <para>
       After receiving <systemitem>SIGTERM</systemitem>, the server
       disallows new connections, but lets existing sessions end their
       work normally. It shuts down only after all of the sessions
       terminate normally. This is the <firstterm>Smart
       Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><systemitem>SIGINT</systemitem><indexterm><primary>SIGINT</></></term>
     <listitem>
      <para>
       The server disallows new connections and sends all existing
       server processes <systemitem>SIGTERM</systemitem>, which will cause them
       to abort their current transactions and exit promptly. It then
       waits for the server processes to exit and finally shuts down. This is the
       <firstterm>Fast Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><systemitem>SIGQUIT</systemitem><indexterm><primary>SIGQUIT</></></term>
     <listitem>
      <para>
      This is the <firstterm>Immediate Shutdown</firstterm>, which
      will cause the <command>postmaster</command> process to send a
      <systemitem>SIGQUIT</systemitem> to all child processes and exit
      immediately, without properly shutting itself down. The child processes
      likewise exit immediately upon receiving
      <systemitem>SIGQUIT</systemitem>. This will lead to recovery (by
      replaying the WAL log) upon next start-up. This is recommended
      only in emergencies.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   The <xref linkend="app-pg-ctl"> program provides a convenient
   interface for sending these signals to shut down the server.
  </para>

  <para>
   Alternatively, you can send the signal directly using <command>kill</>.
   The <acronym>PID</> of the <command>postmaster</command> process can be
   found using the <command>ps</command> program, or from the file
   <filename>postmaster.pid</filename> in the data directory. For
   example, to do a fast shutdown:
<screen>
$ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput>
</screen>
  </para>

   <important>
    <para>
     It is best not to use <systemitem>SIGKILL</systemitem> to shut down
     the server.  Doing so will prevent the server from releasing
     shared memory and semaphores, which may then have to be done
     manually before a new server can be started.  Furthermore,
     <systemitem>SIGKILL</systemitem> kills the <command>postmaster</command>
     process without letting it relay the signal to its subprocesses,
     so it will be necessary to kill the individual subprocesses by hand as
     well.
    </para>
   </important>
 </sect1>

 <sect1 id="encryption-options">
  <title>Encryption Options</title>

  <indexterm zone="encryption-options">
   <primary>encryption</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> offers encryption at several
   levels, and provides flexibility in protecting data from disclosure
   due to database server theft, unscrupulous administrators, and
   insecure networks. Encryption might also be required by government
   regulation, for example, for medical records or financial
   transactions.
  </para>
   
  <variablelist>

  <varlistentry>
   <term>Password Storage Encryption</term>
   <listitem>

    <para>
     By default, database user passwords are stored as MD5 hashes, so
     the administrator can not determine the actual password assigned
     to the user. If MD5 encryption is used for client authentication,
     the unencrypted password is never even temporarily present on the
     server because the client MD5 encrypts it before being sent across
     the network. MD5 is a one-way encryption --- there is no
     decryption algorithm.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Encryption For Specific Columns</term>

   <listitem>
    <para>
     The <filename>/contrib</> function library
     <function>pgcrypto</function> allows certain fields to be stored
     encrypted. This is useful if only some of the data is sensitive.
     The client supplies the decryption key and the data is decrypted
     on the server and then sent to the client.
    </para>
     
    <para>
     The decrypted data and the decryption key are present on the
     server for a brief time while it is being decrypted and
     communicated between the client and server. This presents a brief
     moment where the data and keys can be intercepted by someone with
     complete access to the database server, such as the system
     administrator.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Data Partition Encryption</term>

   <listitem>
    <para>
     On Linux, encryption can be layered on top of a filesystem mount
     using a <quote>loopback device</quote>. This allows an entire
     filesystem partition be encrypted on disk, and decrypted by the
     operating system. On FreeBSD, the equivalent facility is called
     GEOM Based Disk Encryption, or <acronym>gbde</acronym>.
    </para>

    <para>
     This mechanism prevents unecrypted data from being read from the
     drives if the drives or the entire computer is stolen. This
     mechanism does nothing to protect against attacks while the
     filesystem is mounted, because when mounted, the operating system
     provides a unencrypted view of the data. However, to mount the
     filesystem, you need some way for the encryption key to be passed
     to the operating system, and sometimes the key is stored somewhere
     on the host that mounts the disk.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Encrypting Passwords Across A Network</term>

   <listitem>
     <para>
      The <literal>MD5</> authentication method double-encrypts the
      password on the client before sending it to the server. It first
      MD5 encrypts it based on the user name, and then encrypts it
      based on a random salt sent by the server when the database
      connection was made. It is this double-encrypted value that is
      sent over the network to the server. Double-encryption not only
      prevents the password from being discovered, it also prevents
      another connection from replaying the same double-encryption
      value in a later connection.
     </para>
    </listitem>
  </varlistentry>

  <varlistentry>
   <term>Encrypting Data Across A Network</term>

    <listitem>
     <para>
      SSL connections encrypt all data sent across the network: the
      password, the queries, and the data returned. The
      <filename>pg_hba.conf</> file allows administrators to specify
      which hosts can use non-encrypted connections (<literal>host</>)
      and which require SSL-encrypted connections
      (<literal>hostssl</>). Also, clients can specify that they
      connect to servers only via SSL. <application>Stunnel</> or
      <application>SSH</> can also be used to encrypt transmissions.
     </para>
    </listitem>
  </varlistentry>

  <varlistentry>
   <term>SSL Host Authentication</term>

   <listitem>
    <para> 
     It is possible for both the client and server to provide SSL keys
     or certificates to each other. It takes some extra configuration
     on each side, but this provides stronger verification of identity
     than the mere use of passwords. It prevent a computer from
     pretending to be the server just long enough to read the password
     send by the client. It also helps prevent 'man in the middle"
     attacks where a computer between the client and server pretends to
     be the server and reads and passes all data between the client and
     server.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Client-Side Encryption</term>

   <listitem>
    <para>
     If the system administrator can not be trusted, it is necessary
     for the client to encrypt the data; this way, unencrypted data
     never appears on the database server. Data is encrypted on the
     client before being sent to the server, and database results have
     to be decrypted on the client before being used. Peter Wayner's
     book, <citation>Translucent Databases</citation>, discusses how to
     do this in considerable detail.
    </para>
   </listitem>
  </varlistentry>

  </variablelist>

 </sect1>

 <sect1 id="ssl-tcp">
  <title>Secure TCP/IP Connections with SSL</title>

  <indexterm zone="ssl-tcp">
   <primary>SSL</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</> has native support for using
   <acronym>SSL</> connections to encrypt client/server communications
   for increased security. This requires that
   <productname>OpenSSL</productname> is installed on both client and
   server systems and that support in <productname>PostgreSQL</> is
   enabled at build time (see <xref linkend="installation">).
  </para>

  <para>
   With <acronym>SSL</> support compiled in, the
   <productname>PostgreSQL</> server can be started with
   <acronym>SSL</> enabled by setting the parameter
   <xref linkend="guc-ssl"> to <literal>on</> in
   <filename>postgresql.conf</>. When
   starting in <acronym>SSL</> mode, the server will look for the
   files <filename>server.key</> and <filename>server.crt</> in the
   data directory, which must contain the server private key
   and certificate, respectively. These files must be set up correctly
   before an <acronym>SSL</>-enabled server can start. If the private key is
   protected with a passphrase, the server will prompt for the
   passphrase and will not start until it has been entered.
  </para>

  <para>
   The server will listen for both standard and <acronym>SSL</>
   connections on the same TCP port, and will negotiate with any
   connecting client on whether to use <acronym>SSL</>.  By default,
   this is at the client's option; see <xref
   linkend="auth-pg-hba-conf"> about how to set up the server to
   require use of <acronym>SSL</> for some or all connections.
  </para>

  <para>
   For details on how to create your server private key and certificate,
   refer to the <productname>OpenSSL</> documentation. A
   self-signed certificate can be used for testing, but a
   certificate signed by a certificate authority (<acronym>CA</>)
   (either one of the global
   <acronym>CAs</> or a local one) should be used in production so the
   client can verify the server's identity. To create a quick
   self-signed certificate, use the following
   <productname>OpenSSL</productname> command:
<programlisting>
openssl req -new -text -out server.req
</programlisting>
   Fill out the information that <command>openssl</> asks for. Make sure
   that you enter the local host name as <quote>Common Name</>; the challenge
   password can be left blank. The program will generate a key that is
   passphrase protected; it will not accept a passphrase that is less
   than four characters long. To remove the passphrase (as you must if
   you want automatic start-up of the server), run the commands
<programlisting>
openssl rsa -in privkey.pem -out server.key
rm privkey.pem
</programlisting>
   Enter the old passphrase to unlock the existing key. Now do
<programlisting>
openssl req -x509 -in server.req -text -key server.key -out server.crt
chmod og-rwx server.key
</programlisting>
   to turn the certificate into a self-signed certificate and to copy the
   key and certificate to where the server will look for them.
  </para>

  <para>
   If verification of client certificates is required, place the
   certificates of the <acronym>CA</acronym>(s) you wish to check for in
   the file <filename>root.crt</filename> in the data directory.  When
   present, a client certificate will be requested from the client
   during SSL connection startup, and it must have been signed by one of the
   certificates present in <filename>root.crt</filename>.
  </para>

  <para>
   When the <filename>root.crt</filename> file is not present, client
   certificates will not be requested or checked.  In this mode, SSL
   provides communication security but not authentication.
  </para>

  <para>
   The files <filename>server.key</>, <filename>server.crt</>,
   and <filename>root.crt</filename> are only examined during server
   start; so you must restart the server to make changes in them take
   effect.
  </para>
 </sect1>

 <sect1 id="ssh-tunnels">
  <title>Secure TCP/IP Connections with <application>SSH</application> Tunnels</title>

  <indexterm zone="ssh-tunnels">
   <primary>ssh</primary>
  </indexterm>

  <para>
   One can use <application>SSH</application> to encrypt the network
   connection between clients and a
   <productname>PostgreSQL</productname> server. Done properly, this
   provides an adequately secure network connection, even for non-SSL-capable
   clients.
  </para>

  <para>
   First make sure that an <application>SSH</application> server is
   running properly on the same machine as the
   <productname>PostgreSQL</productname> server and that you can log in using
   <command>ssh</command> as some user. Then you can establish a secure
   tunnel with a command like this from the client machine:
<programlisting>
ssh -L 3333:foo.com:5432 joe@foo.com
</programlisting>
   The first number in the <option>-L</option> argument, 3333, is the
   port number of your end of the tunnel; it can be chosen freely. The
   second number, 5432, is the remote end of the tunnel: the port
   number your server is using. The name or IP address between
   the port numbers is the host with the database server you are going
   to connect to. In order to connect to the database server using
   this tunnel, you connect to port 3333 on the local machine:
<programlisting>
psql -h localhost -p 3333 template1
</programlisting>
   To the database server it will then look as though you are really
   user <literal>joe@foo.com</literal> and it will use whatever
   authentication procedure was configured for connections from this
   user and host.  Note that the server will not think the connection is
   SSL-encrypted, since in fact it is not encrypted between the
   <application>SSH</application> server and the
   <productname>PostgreSQL</productname> server.  This should not pose any
   extra security risk as long as they are on the same machine.
  </para>
  <para>
   In order for the
   tunnel setup to succeed you must be allowed to connect via
   <command>ssh</command> as <literal>joe@foo.com</literal>, just
   as if you had attempted to use <command>ssh</command> to set up a
   terminal session.
  </para>

  <tip>
   <para>
    Several other applications exist that can provide secure tunnels using
    a procedure similar in concept to the one just described.
   </para>
  </tip>

 </sect1>

</chapter>

<!-- Keep this comment at the end of the file
Local variables:
mode:sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"./reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:("/usr/lib/sgml/catalog")
sgml-local-ecat-files:nil
End:
-->
