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$PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.57.4.2 2005/10/06 21:21:27 neilc Exp $
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<chapter id="plpgsql"> 
  <title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>

 <indexterm zone="plpgsql">
  <primary>PL/pgSQL</primary>
 </indexterm>

 <para>
  <application>PL/pgSQL</application> is a loadable procedural
  language for the <productname>PostgreSQL</productname> database
  system.  The design goals of <application>PL/pgSQL</> were to create
  a loadable procedural language that

    <itemizedlist>
     <listitem>
      <para>
       can be used to create functions and trigger procedures,
      </para>
     </listitem>
     <listitem>
      <para>
       adds control structures to the <acronym>SQL</acronym> language,
      </para>
     </listitem>
     <listitem>
      <para>
       can perform complex computations,
      </para>
     </listitem>
     <listitem>
      <para>
       inherits all user-defined types, functions, and operators,
      </para>
     </listitem>
     <listitem>
      <para>
       can be defined to be trusted by the server,
      </para>
     </listitem>
     <listitem>
      <para>
       is easy to use.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Except for input/output conversion and calculation functions
    for user-defined types, anything that can be defined in C language
    functions can also be done with <application>PL/pgSQL</application>.
    For example, it is possible to
    create complex conditional computation functions and later use
    them to define operators or use them in index expressions.
   </para>

  <sect1 id="plpgsql-overview">
   <title>Overview</title>

   <para>
    The <application>PL/pgSQL</> call handler parses the function's source text and
    produces an internal binary instruction tree the first time the
    function is called (within each session).  The instruction tree
    fully translates the 
    <application>PL/pgSQL</> statement structure, but individual
    <acronym>SQL</acronym> expressions and <acronym>SQL</acronym> commands
    used in the function are not translated immediately.
   </para>

   <para>
    As each expression and <acronym>SQL</acronym> command is first
    used in the function, the <application>PL/pgSQL</> interpreter
    creates a prepared execution plan (using the
    <acronym>SPI</acronym> manager's <function>SPI_prepare</function>
    and <function>SPI_saveplan</function>
    functions).<indexterm><primary>preparing a query</><secondary>in
    PL/pgSQL</></> Subsequent visits to that expression or command
    reuse the prepared plan.  Thus, a function with conditional code
    that contains many statements for which execution plans might be
    required will only prepare and save those plans that are really
    used during the lifetime of the database connection.  This can
    substantially reduce the total amount of time required to parse,
    and generate execution plans for the statements in a
    <application>PL/pgSQL</> function. A disadvantage is that errors
    in a specific expression or command may not be detected until that
    part of the function is reached in execution.
   </para>

   <para>
    Once <application>PL/pgSQL</> has made an execution plan for a particular
    command in a function, it will reuse that plan for the life of the
    database connection.  This is usually a win for performance, but it
    can cause some problems if you dynamically
    alter your database schema. For example:

<programlisting>
CREATE FUNCTION populate() RETURNS integer AS $$
DECLARE
    -- declarations
BEGIN
    PERFORM my_function();
END;
$$ LANGUAGE plpgsql;
</programlisting>

    If you execute the above function, it will reference the OID for
    <function>my_function()</function> in the execution plan produced for
    the <command>PERFORM</command> statement. Later, if you
    drop and recreate <function>my_function()</function>, then
    <function>populate()</function> will not be able to find
    <function>my_function()</function> anymore. You would then have to
    recreate <function>populate()</function>, or at least start a new
    database session so that it will be compiled afresh. Another way
    to avoid this problem is to use <command>CREATE OR REPLACE
    FUNCTION</command> when updating the definition of
    <function>my_function</function> (when a function is
    <quote>replaced</quote>, its OID is not changed).
   </para>

   <para>
    Because <application>PL/pgSQL</application> saves execution plans
        in this way, SQL commands that appear directly in a
        <application>PL/pgSQL</application> function must refer to the
        same tables and columns on every execution; that is, you cannot use
        a parameter as the name of a table or column in an SQL command.  To get
        around this restriction, you can construct dynamic commands using
        the <application>PL/pgSQL</application> <command>EXECUTE</command>
        statement &mdash; at the price of constructing a new execution plan on
        every execution.
   </para>

   <note>
        <para>
         The <application>PL/pgSQL</application>
         <command>EXECUTE</command> statement is not related to the
         <xref linkend="sql-execute" endterm="sql-execute-title"> SQL
         statement supported by the
         <productname>PostgreSQL</productname> server. The server's
         <command>EXECUTE</command> statement cannot be used within
         <application>PL/pgSQL</> functions (and is not needed).
        </para>
   </note>

  <sect2 id="plpgsql-advantages">
   <title>Advantages of Using <application>PL/pgSQL</application></title>

    <para>
     <acronym>SQL</acronym> is the language <productname>PostgreSQL</>
     and most other relational databases use as query language. It's
     portable and easy to learn. But every <acronym>SQL</acronym>
     statement must be executed individually by the database server.
    </para>

    <para>
     That means that your client application must send each query to
     the database server, wait for it to be processed, receive the
     results, do some computation, then send other queries to the
     server. All this incurs interprocess communication and may also
     incur network overhead if your client is on a different machine
     than the database server.
    </para>

    <para>
     With <application>PL/pgSQL</application> you can group a block of computation and a
     series of queries <emphasis>inside</emphasis> the
     database server, thus having the power of a procedural
     language and the ease of use of SQL, but saving lots of
     time because you don't have the whole client/server
     communication overhead. This can make for a
     considerable performance increase.
    </para>

    <para>
     Also, with <application>PL/pgSQL</application> you can use all
     the data types, operators and functions of SQL.
    </para>
  </sect2>

  <sect2 id="plpgsql-args-results">
   <title>Supported Argument and Result Data Types</title>

    <para>
     Functions written in <application>PL/pgSQL</application> can accept
     as arguments any scalar or array data type supported by the server,
     and they can return a result of any of these types.  They can also
     accept or return any composite type (row type) specified by name.
     It is also possible to declare a <application>PL/pgSQL</application>
     function as returning <type>record</>, which means that the result
     is a row type whose columns are determined by specification in the
     calling query, as discussed in <xref linkend="queries-tablefunctions">.
    </para>

    <para>
     <application>PL/pgSQL</> functions may also be declared to accept
     and return the polymorphic types
     <type>anyelement</type> and <type>anyarray</type>.  The actual
     data types handled by a polymorphic function can vary from call to
     call, as discussed in <xref linkend="extend-types-polymorphic">.
     An example is shown in <xref linkend="plpgsql-declaration-aliases">.
    </para>

    <para>
     <application>PL/pgSQL</> functions can also be declared to return
     a <quote>set</>, or table, of any data type they can return a single
     instance of.  Such a function generates its output by executing
     <literal>RETURN NEXT</> for each desired element of the result set.
    </para>

    <para>
     Finally, a <application>PL/pgSQL</> function may be declared to return
     <type>void</> if it has no useful return value.
    </para>

    <para>
     <application>PL/pgSQL</> does not currently have full support for
     domain types: it treats a domain the same as the underlying scalar
     type.  This means that constraints associated with the domain will
     not be enforced.  This is not an issue for function arguments, but
     it is a hazard if you declare a <application>PL/pgSQL</> function
     as returning a domain type.
    </para>
  </sect2>
 </sect1>

 <sect1 id="plpgsql-development-tips">
  <title>Tips for Developing in <application>PL/pgSQL</application></title>

   <para>
    One good way to develop in
    <application>PL/pgSQL</> is to use the text editor of your
    choice to create your functions, and in another window, use
    <application>psql</application> to load and test those functions.
    If you are doing it this way, it
    is a good idea to write the function using <command>CREATE OR
    REPLACE FUNCTION</>. That way you can just reload the file to update
    the function definition.  For example:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
          ....
$$ LANGUAGE plpgsql;
</programlisting>
   </para>

   <para>
    While running <application>psql</application>, you can load or reload such
    a function definition file with
<programlisting>
\i filename.sql
</programlisting>
    and then immediately issue SQL commands to test the function.
   </para>

   <para>
    Another good way to develop in <application>PL/pgSQL</> is with a
    GUI database access tool that facilitates development in a
    procedural language. One example of such as a tool is
    <application>PgAccess</>, although others exist. These tools often
    provide convenient features such as escaping single quotes and
    making it easier to recreate and debug functions.
   </para>

  <sect2 id="plpgsql-quote-tips">
   <title>Handling of Quotation Marks</title>

   <para>
    The code of a <application>PL/pgSQL</> function is specified in
    <command>CREATE FUNCTION</command> as a string literal.  If you
    write the string literal in the ordinary way with surrounding
    single quotes, then any single quotes inside the function body
    must be doubled; likewise any backslashes must be doubled.
    Doubling quotes is at best tedious, and in more complicated cases
    the code can become downright incomprehensible, because you can
    easily find yourself needing half a dozen or more adjacent quote marks.
    It's recommended that you instead write the function body as a
    <quote>dollar-quoted</> string literal (see <xref
    linkend="sql-syntax-dollar-quoting">).  In the dollar-quoting
    approach, you never double any quote marks, but instead take care to
    choose a different dollar-quoting delimiter for each level of
    nesting you need.  For example, you might write the <command>CREATE
    FUNCTION</command> command as
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
          ....
$PROC$ LANGUAGE plpgsql;
</programlisting>
    Within this, you might use quote marks for simple literal strings in
    SQL commands and <literal>$$</> to delimit fragments of SQL commands
    that you are assembling as strings.  If you need to quote text that
    includes <literal>$$</>, you could use <literal>$Q$</>, and so on.
   </para>

   <para>
    The following chart shows what you have to do when writing quote
    marks without dollar quoting.  It may be useful when translating
    pre-dollar quoting code into something more comprehensible.
  </para>

  <variablelist>
   <varlistentry>
    <term>1 quotation mark</term>
    <listitem>
     <para>
      To begin and end the function body, for example:
<programlisting>
CREATE FUNCTION foo() RETURNS integer AS '
          ....
' LANGUAGE plpgsql;
</programlisting>
      Anywhere within a single-quoted function body, quote marks
      <emphasis>must</> appear in pairs.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>2 quotation marks</term>
    <listitem>
     <para>
      For string literals inside the function body, for example:
<programlisting>
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting>
      In the dollar-quoting approach, you'd just write
<programlisting>
a_output := 'Blah';
SELECT * FROM users WHERE f_name='foobar';
</programlisting>
      which is exactly what the <application>PL/pgSQL</> parser would see
      in either case.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>4 quotation marks</term>
    <listitem>
     <para>
      When you need a single quotation mark in a string constant inside the
      function body, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
</programlisting>
      The value actually appended to <literal>a_output</literal> would be:
      <literal> AND name LIKE 'foobar' AND xyz</literal>.
     </para>
     <para>
      In the dollar-quoting approach, you'd write
<programlisting>
a_output := a_output || $$ AND name LIKE 'foobar' AND xyz$$
</programlisting>
      being careful that any dollar-quote delimiters around this are not
      just <literal>$$</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>6 quotation marks</term>
    <listitem>
     <para>
      When a single quotation mark in a string inside the function body is
      adjacent to the end of that string constant, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar''''''
</programlisting>
      The value appended to <literal>a_output</literal> would then be:
      <literal> AND name LIKE 'foobar'</literal>.
     </para>
     <para>
      In the dollar-quoting approach, this becomes
<programlisting>
a_output := a_output || $$ AND name LIKE 'foobar'$$
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>10 quotation marks</term>
    <listitem>
     <para>
      When you want two single quotation marks in a string constant (which
      accounts for 8 quotation marks) and this is adjacent to the end of that
      string constant (2 more).  You will probably only need that if
      you are writing a function that generates other functions, as in
      <xref linkend="plpgsql-porting-ex2">.
      For example:
<programlisting>
a_output := a_output || '' if v_'' || 
    referrer_keys.kind || '' like '''''''''' 
    || referrer_keys.key_string || '''''''''' 
    then return ''''''  || referrer_keys.referrer_type 
    || ''''''; end if;''; 
</programlisting>
      The value of <literal>a_output</literal> would then be:
<programlisting>
if v_... like ''...'' then return ''...''; end if;
</programlisting>
     </para>
     <para>
      In the dollar-quoting approach, this becomes
<programlisting>
a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
    || referrer_keys.key_string || $$'
    then return '$$  || referrer_keys.referrer_type 
    || $$'; end if;$$; 
</programlisting>
      where we assume we only need to put single quote marks into
      <literal>a_output</literal>, because it will be re-quoted before use.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

   <para>
    A variant approach is to escape quotation marks in the function body
    with a backslash rather than by doubling them.  With this method
    you'll find yourself writing things like <literal>\'\'</> instead
    of <literal>''''</>.  Some find this easier to keep track of, some
    do not.
   </para>
  </sect2>
 </sect1>

 <sect1 id="plpgsql-structure">
  <title>Structure of <application>PL/pgSQL</application></title>

  <para>
   <application>PL/pgSQL</application> is a block-structured language.
   The complete text of a function definition must be a
   <firstterm>block</>. A block is defined as:

<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
    <replaceable>declarations</replaceable> </optional>
BEGIN
    <replaceable>statements</replaceable>
END;
</synopsis>
    </para>

    <para>
     Each declaration and each statement within a block is terminated
     by a semicolon.  A block that appears within another block must
     have a semicolon after <literal>END</literal>, as shown above;
     however the final <literal>END</literal> that
     concludes a function body does not require a semicolon.
    </para>

    <para>
     All key words and identifiers can be written in mixed upper and
     lower case.  Identifiers are implicitly converted to lowercase
     unless double-quoted.
    </para>

    <para>
     There are two types of comments in <application>PL/pgSQL</>. A double
     dash (<literal>--</literal>) starts a comment that extends to the end of
     the line. A <literal>/*</literal> starts a block comment that extends to
     the next occurrence of <literal>*/</literal>.  Block comments cannot be
     nested, but double dash comments can be enclosed into a block comment and
     a double dash can hide the block comment delimiters <literal>/*</literal>
     and <literal>*/</literal>.
    </para>

    <para>
     Any statement in the statement section of a block
     can be a <firstterm>subblock</>.  Subblocks can be used for
     logical grouping or to localize variables to a small group
     of statements.
    </para>

    <para>
     The variables declared in the declarations section preceding a
     block are initialized to their default values every time the
     block is entered, not only once per function call. For example:
<programlisting>
CREATE FUNCTION somefunc() RETURNS integer AS $$
DECLARE
    quantity integer := 30;
BEGIN
    RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 30
    quantity := 50;
    --
    -- Create a subblock
    --
    DECLARE
        quantity integer := 80;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 80
    END;

    RAISE NOTICE 'Quantity here is %', quantity;  -- Quantity here is 50

    RETURN quantity;
END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>

    <para>
     It is important not to confuse the use of
     <command>BEGIN</>/<command>END</> for grouping statements in
     <application>PL/pgSQL</> with the database commands for transaction
     control.  <application>PL/pgSQL</>'s <command>BEGIN</>/<command>END</>
     are only for grouping; they do not start or end a transaction.
     Functions and trigger procedures are always executed within a transaction
     established by an outer query &mdash; they cannot start or commit that
     transaction, since there would be no context for them to execute in.
     However, a block containing an <literal>EXCEPTION</> clause effectively
     forms a subtransaction that can be rolled back without affecting the
     outer transaction.  For more about that see <xref
     linkend="plpgsql-error-trapping">.
    </para>
  </sect1>

  <sect1 id="plpgsql-declarations">
    <title>Declarations</title>

    <para>
     All variables used in a block must be declared in the
     declarations section of the block. 
     (The only exception is that the loop variable of a <literal>FOR</> loop
     iterating over a range of integer values is automatically declared as an
     integer variable.)
    </para>

    <para>
     <application>PL/pgSQL</> variables can have any SQL data type, such as
     <type>integer</type>, <type>varchar</type>, and
     <type>char</type>.
    </para>

    <para>
     Here are some examples of variable declarations:
<programlisting>
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
</programlisting>
    </para>

    <para>
     The general syntax of a variable declaration is:
<synopsis>
<replaceable>name</replaceable> <optional> CONSTANT </optional> <replaceable>type</replaceable> <optional> NOT NULL </optional> <optional> { DEFAULT | := } <replaceable>expression</replaceable> </optional>;
</synopsis>
      The <literal>DEFAULT</> clause, if given, specifies the initial value assigned
      to the variable when the block is entered.  If the <literal>DEFAULT</> clause
      is not given then the variable is initialized to the
      <acronym>SQL</acronym> null value. 
      The <literal>CONSTANT</> option prevents the variable from being assigned to,
      so that its value remains constant for the duration of the block.
      If <literal>NOT NULL</>
      is specified, an assignment of a null value results in a run-time
      error. All variables declared as <literal>NOT NULL</>
      must have a nonnull default value specified.
     </para>

     <para>
      The default value is evaluated every time the block is entered. So,
      for example, assigning <literal>now()</literal> to a variable of type
      <type>timestamp</type> causes the variable to have the
      time of the current function call, not the time when the function was
      precompiled.
     </para>

     <para>
      Examples:
<programlisting>
quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
user_id CONSTANT integer := 10;
</programlisting>
     </para>

    <sect2 id="plpgsql-declaration-aliases">
     <title>Aliases for Function Parameters</title>

     <para>
      Parameters passed to functions are named with the identifiers
      <literal>$1</literal>, <literal>$2</literal>,
      etc.  Optionally, aliases can be declared for
      <literal>$<replaceable>n</replaceable></literal>
      parameter names for increased readability.  Either the alias or the
      numeric identifier can then be used to refer to the parameter value.
     </para>

     <para>
      There are two ways to create an alias.  The preferred way is to give a
      name to the parameter in the <command>CREATE FUNCTION</command> command,
      for example:
<programlisting>
CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
BEGIN
    RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
</programlisting>
      The other way, which was the only way available before
      <productname>PostgreSQL</productname> 8.0, is to explicitly
      declare an alias, using the declaration syntax

<synopsis>
<replaceable>name</replaceable> ALIAS FOR $<replaceable>n</replaceable>;
</synopsis>

      The same example in this style looks like
<programlisting>
CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
    subtotal ALIAS FOR $1;
BEGIN
    RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
</programlisting>
      Some more examples:
<programlisting>
CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
DECLARE
    v_string ALIAS FOR $1;
    index ALIAS FOR $2;
BEGIN
    -- some computations here
END;
$$ LANGUAGE plpgsql;


CREATE FUNCTION concat_selected_fields(in_t tablename) RETURNS text AS $$
BEGIN
    RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
$$ LANGUAGE plpgsql;
</programlisting>
     </para>

     <para>
      When the return type of a <application>PL/pgSQL</application>
      function is declared as a polymorphic type (<type>anyelement</type>
      or <type>anyarray</type>), a special parameter <literal>$0</literal>
      is created.  Its data type is the actual return type of the function,
      as deduced from the actual input types (see <xref
      linkend="extend-types-polymorphic">).
      This allows the function to access its actual return type
      as shown in <xref linkend="plpgsql-declaration-type">.
      <literal>$0</literal> is initialized to null and can be modified by
      the function, so it can be used to hold the return value if desired,
      though that is not required.  <literal>$0</literal> can also be
      given an alias.  For example, this function works on any data type
      that has a <literal>+</> operator:
<programlisting>
CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
RETURNS anyelement AS $$
DECLARE
    result ALIAS FOR $0;
BEGIN
    result := v1 + v2 + v3;
    RETURN result;
END;
$$ LANGUAGE plpgsql;
</programlisting>
     </para>
    </sect2>

  <sect2 id="plpgsql-declaration-type">
   <title>Copying Types</title>

<synopsis>
<replaceable>variable</replaceable>%TYPE
</synopsis>

   <para>
    <literal>%TYPE</literal> provides the data type of a variable or
    table column. You can use this to declare variables that will hold
    database values. For example, let's say you have a column named
    <literal>user_id</literal> in your <literal>users</literal>
    table. To declare a variable with the same data type as
    <literal>users.user_id</> you write:
<programlisting>
user_id users.user_id%TYPE;
</programlisting>
   </para>

   <para>
    By using <literal>%TYPE</literal> you don't need to know the data
    type of the structure you are referencing, and most importantly,
    if the data type of the referenced item changes in the future (for
    instance: you change the type of <literal>user_id</>
    from <type>integer</type> to <type>real</type>), you may not need
    to change your function definition.
   </para>

   <para>
    <literal>%TYPE</literal> is particularly valuable in polymorphic
    functions, since the data types needed for internal variables may
    change from one call to the next.  Appropriate variables can be
    created by applying <literal>%TYPE</literal> to the function's
    arguments or result placeholders.
   </para>

  </sect2>

    <sect2 id="plpgsql-declaration-rowtypes">
     <title>Row Types</title>

<synopsis>
<replaceable>name</replaceable> <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>;
<replaceable>name</replaceable> <replaceable>composite_type_name</replaceable>;
</synopsis>

   <para>
    A variable of a composite type is called a <firstterm>row</>
    variable (or <firstterm>row-type</> variable).  Such a variable
    can hold a whole row of a <command>SELECT</> or <command>FOR</>
    query result, so long as that query's column set matches the
    declared type of the variable.
    The individual fields of the row value
    are accessed using the usual dot notation, for example
    <literal>rowvar.field</literal>.
   </para>

   <para>
    A row variable can be declared to have the same type as the rows of
    an existing table or view, by using the
    <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>
    notation; or it can be declared by giving a composite type's name.
    (Since every table has an associated composite type of the same name,
    it actually does not matter in <productname>PostgreSQL</> whether you
    write <literal>%ROWTYPE</literal> or not.  But the form with
    <literal>%ROWTYPE</literal> is more portable.)
   </para>

   <para>
    Parameters to a function can be
    composite types (complete table rows). In that case, the
    corresponding identifier <literal>$<replaceable>n</replaceable></> will be a row variable, and fields can
    be selected from it, for example <literal>$1.user_id</literal>.
   </para>

   <para>
    Only the user-defined columns of a table row are accessible in a
    row-type variable, not the OID or other system columns (because the
    row could be from a view).  The fields of the row type inherit the
    table's field size or precision for data types such as
    <type>char(<replaceable>n</>)</type>.
   </para>

   <para>
    Here is an example of using composite types:
<programlisting>
CREATE FUNCTION merge_fields(t_row tablename) RETURNS text AS $$
DECLARE
    t2_row table2name%ROWTYPE;
BEGIN
    SELECT * INTO t2_row FROM table2name WHERE ... ;
    RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
END;
$$ LANGUAGE plpgsql;

SELECT merge_fields(t.*) FROM tablename t WHERE ... ;
</programlisting>
   </para>
  </sect2>

  <sect2 id="plpgsql-declaration-records">
   <title>Record Types</title>

<synopsis>
<replaceable>name</replaceable> RECORD;
</synopsis>

   <para>
    Record variables are similar to row-type variables, but they have no
    predefined structure.  They take on the actual row structure of the
    row they are assigned during a <command>SELECT</> or <command>FOR</> command.  The substructure
    of a record variable can change each time it is assigned to.
    A consequence of this is that until a record variable is first assigned
    to, it has no substructure, and any attempt to access a
    field in it will draw a run-time error.
   </para>

   <para>
    Note that <literal>RECORD</> is not a true data type, only a placeholder.
    One should also realize that when a <application>PL/pgSQL</application>
    function is declared to return type <type>record</>, this is not quite the
    same concept as a record variable, even though such a function may well
    use a record variable to hold its result.  In both cases the actual row
    structure is unknown when the function is written, but for a function
    returning <type>record</> the actual structure is determined when the
    calling query is parsed, whereas a record variable can change its row
    structure on-the-fly.
   </para>
  </sect2>

  <sect2 id="plpgsql-declaration-renaming-vars">
   <title><literal>RENAME</></title>

<synopsis>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</synopsis>

   <para>
    Using the <literal>RENAME</literal> declaration you can change the
    name of a variable, record or row. This is primarily useful if
    <varname>NEW</varname> or <varname>OLD</varname> should be
    referenced by another name inside a trigger procedure.  See also
    <literal>ALIAS</literal>.
   </para>

   <para>
    Examples:
<programlisting>
RENAME id TO user_id;
RENAME this_var TO that_var;
</programlisting>
   </para>

    <note>
     <para>
      <literal>RENAME</literal> appears to be broken as of
      <productname>PostgreSQL</> 7.3.  Fixing this is of low priority,
      since <literal>ALIAS</literal> covers most of the practical uses
      of <literal>RENAME</literal>.
     </para>
    </note>
   </sect2>
  </sect1>

  <sect1 id="plpgsql-expressions">
  <title>Expressions</title>

    <para>
     All expressions used in <application>PL/pgSQL</application>
     statements are processed using the server's regular
     <acronym>SQL</acronym> executor.  In effect, a query like
<synopsis>
SELECT <replaceable>expression</replaceable>
</synopsis>
     is executed using the <acronym>SPI</acronym> manager. Before evaluation,
     occurrences of <application>PL/pgSQL</application> variable
     identifiers are replaced by parameters, and the actual values from
     the variables are passed to the executor in the parameter array.
     This allows the query plan for the <command>SELECT</command> to
     be prepared just once and then reused for subsequent
     evaluations.
    </para>

    <para>
     The evaluation done by the <productname>PostgreSQL</productname>
     main parser has some side
     effects on the interpretation of constant values. In detail there
     is a difference between what these two functions do:

<programlisting>
CREATE FUNCTION logfunc1(logtxt text) RETURNS timestamp AS $$
    BEGIN
        INSERT INTO logtable VALUES (logtxt, 'now');
        RETURN 'now';
    END;
$$ LANGUAGE plpgsql;
</programlisting>

     and

<programlisting>
CREATE FUNCTION logfunc2(logtxt text) RETURNS timestamp AS $$
    DECLARE
        curtime timestamp;
    BEGIN
        curtime := 'now';
        INSERT INTO logtable VALUES (logtxt, curtime);
        RETURN curtime;
    END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>

    <para>
     In the case of <function>logfunc1</function>, the 
     <productname>PostgreSQL</productname> main parser knows when 
     preparing the plan for the <command>INSERT</command>, that the string 
     <literal>'now'</literal> should be interpreted as 
     <type>timestamp</type> because the target column of <classname>logtable</classname>
     is of that type. Thus, it will make a constant from it at this
     time and this constant value is then used in all invocations of 
     <function>logfunc1</function> during the lifetime of the
     session. Needless to say that this isn't what the
     programmer wanted.
    </para>

    <para>
     In the case of <function>logfunc2</function>, the 
     <productname>PostgreSQL</productname> main parser does not know
     what type <literal>'now'</literal> should become and therefore 
     it returns a data value of type <type>text</type> containing the string 
     <literal>now</literal>. During the ensuing assignment
     to the local variable <varname>curtime</varname>, the
     <application>PL/pgSQL</application> interpreter casts this
     string to the <type>timestamp</type> type by calling the
     <function>text_out</function> and <function>timestamp_in</function>
     functions for the conversion.  So, the computed time stamp is updated
     on each execution as the programmer expects.
    </para>

    <para>
     The mutable nature of record variables presents a problem in this
     connection.  When fields of a record variable are used in
     expressions or statements, the data types of the fields must not
     change between calls of one and the same expression, since the
     expression will be planned using the data type that is present
     when the expression is first reached.  Keep this in mind when
     writing trigger procedures that handle events for more than one
     table.  (<command>EXECUTE</command> can be used to get around
     this problem when necessary.)
    </para>
  </sect1>

  <sect1 id="plpgsql-statements">
  <title>Basic Statements</title>

   <para>
    In this section and the following ones, we describe all the statement
    types that are explicitly understood by
    <application>PL/pgSQL</application>.
    Anything not recognized as one of these statement types is presumed
    to be an SQL command and is sent to the main database engine to execute
    (after substitution of any <application>PL/pgSQL</application> variables
    used in the statement).  Thus,
    for example, the SQL commands <command>INSERT</>, <command>UPDATE</>, and
    <command>DELETE</> may be considered to be statements of
    <application>PL/pgSQL</application>, but they are not specifically
    listed here.
   </para>

   <sect2 id="plpgsql-statements-assignment">
    <title>Assignment</title>

    <para>
     An assignment of a value to a variable or row/record field is
     written as:
<synopsis>
<replaceable>identifier</replaceable> := <replaceable>expression</replaceable>;
</synopsis>
     As explained above, the expression in such a statement is evaluated
     by means of an SQL <command>SELECT</> command sent to the main
     database engine.  The expression must yield a single value.
    </para>

    <para>
     If the expression's result data type doesn't match the variable's
     data type, or the variable has a specific size/precision
     (like <type>char(20)</type>), the result value will be implicitly
     converted by the <application>PL/pgSQL</application> interpreter using
     the result type's output-function and 
     the variable type's input-function. Note that this could potentially
     result in run-time errors generated by the input function, if the
     string form of the result value is not acceptable to the input function.
    </para>

    <para>
     Examples:
<programlisting>
user_id := 20;
tax := subtotal * 0.06;
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-select-into">
    <title><command>SELECT INTO</command></title>

    <indexterm zone="plpgsql-select-into">
     <primary>SELECT INTO</primary>
     <secondary>in PL/pgSQL</secondary>
    </indexterm>

    <para>
     The result of a <command>SELECT</command> command yielding multiple columns (but
     only one row) can be assigned to a record variable, row-type
     variable, or list of scalar variables.  This is done by:

<synopsis>
SELECT INTO <replaceable>target</replaceable> <replaceable>select_expressions</replaceable> FROM ...;
</synopsis>

     where <replaceable>target</replaceable> can be a record variable, a row
     variable, or a comma-separated list of simple variables and
     record/row fields.  The <replaceable>select_expressions</replaceable>
     and the remainder of the command are the same as in regular SQL.
    </para>

    <para>
     Note that this is quite different from
     <productname>PostgreSQL</>'s normal interpretation of
     <command>SELECT INTO</command>, where the <literal>INTO</> target
     is a newly created table.  If you want to create a table from a
     <command>SELECT</> result inside a
     <application>PL/pgSQL</application> function, use the syntax
     <command>CREATE TABLE ... AS SELECT</command>.
    </para>

    <para>
     If a row or a variable list is used as target, the selected values
     must exactly match the structure of the target, or a run-time error
     occurs.  When a record variable is the target, it automatically
     configures itself to the row type of the query result columns.
    </para>

    <para>
     Except for the <literal>INTO</> clause, the <command>SELECT</>
     statement is the same as a normal SQL <command>SELECT</> command
     and can use its full power.
    </para>

    <para>
     The <literal>INTO</> clause can appear almost anywhere in the
     <command>SELECT</command> statement.  Customarily it is written
     either just after <literal>SELECT</> as shown above, or
     just before <literal>FROM</> &mdash; that is, either just before
     or just after the list of <replaceable>select_expressions</replaceable>.
    </para>

    <para>
     If the query returns zero rows, null values are assigned to the
     target(s).  If the query returns multiple rows, the first
     row is assigned to the target(s) and the rest are discarded.
     (Note that <quote>the first row</> is not well-defined unless you've
     used <literal>ORDER BY</>.)
    </para>

    <para>
     You can check the special <literal>FOUND</literal> variable (see
     <xref linkend="plpgsql-statements-diagnostics">) after a
     <command>SELECT INTO</command> statement to determine whether the
     assignment was successful, that is, at least one row was was returned by
     the query. For example:

<programlisting>
SELECT INTO myrec * FROM emp WHERE empname = myname;
IF NOT FOUND THEN
    RAISE EXCEPTION 'employee % not found', myname;
END IF;
</programlisting>
    </para>

    <para>
     To test for whether a record/row result is null, you can use the
     <literal>IS NULL</literal> conditional.  There is, however, no
     way to tell whether any additional rows might have been
     discarded.  Here is an example that handles the case where no
     rows have been returned:
<programlisting>
DECLARE
    users_rec RECORD;
BEGIN
    SELECT INTO users_rec * FROM users WHERE user_id=3;

    IF users_rec.homepage IS NULL THEN
        -- user entered no homepage, return "http://"
        RETURN 'http://';
    END IF;
END;
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-perform">
    <title>Executing an Expression or Query With No Result</title>

    <para>
     Sometimes one wishes to evaluate an expression or query but
     discard the result (typically because one is calling a function
     that has useful side-effects but no useful result value).  To do
     this in <application>PL/pgSQL</application>, use the
     <command>PERFORM</command> statement:

<synopsis>
PERFORM <replaceable>query</replaceable>;
</synopsis>

     This executes <replaceable>query</replaceable> and discards the
     result.  Write the <replaceable>query</replaceable> the same
     way as you would in an SQL <command>SELECT</> command, but replace the
     initial keyword <command>SELECT</> with <command>PERFORM</command>.
     <application>PL/pgSQL</application> variables will be
     substituted into the query as usual.  Also, the special variable
     <literal>FOUND</literal> is set to true if the query produced at
     least one row or false if it produced no rows.
    </para>

    <note>
     <para>
      One might expect that <command>SELECT</command> with no
      <literal>INTO</> clause would accomplish this result, but at
      present the only accepted way to do it is
      <command>PERFORM</command>.
     </para>
    </note>

    <para>
     An example:
<programlisting>
PERFORM create_mv('cs_session_page_requests_mv', my_query);
</programlisting>
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-null">
    <title>Doing Nothing At All</title>

    <para>
     Sometimes a placeholder statement that does nothing is useful.
     For example, it can indicate that one arm of an if/then/else
     chain is deliberately empty.  For this purpose, use the
     <command>NULL</command> statement:

<synopsis>
NULL;
</synopsis>
    </para>

    <para>
     For example, the following two fragments of code are equivalent:
<programlisting>
    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            NULL;  -- ignore the error
    END;
</programlisting>

<programlisting>
    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN  -- ignore the error
    END;
</programlisting>
     Which is preferable is a matter of taste.
    </para>

    <note>
     <para>
      In Oracle's PL/SQL, empty statement lists are not allowed, and so
      <command>NULL</> statements are <emphasis>required</> for situations
      such as this.  <application>PL/pgSQL</application> allows you to
      just write nothing, instead.
     </para>
    </note>

   </sect2>

   <sect2 id="plpgsql-statements-executing-dyn">
    <title>Executing Dynamic Commands</title>

    <para>
     Oftentimes you will want to generate dynamic commands inside your
     <application>PL/pgSQL</application> functions, that is, commands
     that will involve different tables or different data types each
     time they are executed.  <application>PL/pgSQL</application>'s
     normal attempts to cache plans for commands will not work in such
     scenarios.  To handle this sort of problem, the
     <command>EXECUTE</command> statement is provided:

<synopsis>
EXECUTE <replaceable class="command">command-string</replaceable>;
</synopsis>

     where <replaceable>command-string</replaceable> is an expression
     yielding a string (of type
     <type>text</type>) containing the command
     to be executed.  This string is fed literally to the SQL engine.
    </para>

    <para>
     Note in particular that no substitution of <application>PL/pgSQL</>
     variables is done on the command string.  The values of variables must
     be inserted in the command string as it is constructed.
    </para>

    <para>
     Unlike all other commands in <application>PL/pgSQL</>, a command
     run by an <command>EXECUTE</command> statement is not prepared
     and saved just once during the life of the session.  Instead, the
     command is prepared each time the statement is run. The command
     string can be dynamically created within the function to perform
     actions on different tables and columns.
    </para>

    <para>
     The results from <command>SELECT</command> commands are discarded
     by <command>EXECUTE</command>, and <command>SELECT INTO</command>
     is not currently supported within <command>EXECUTE</command>.
     So there is no way to extract a result from a dynamically-created
     <command>SELECT</command> using the plain <command>EXECUTE</command>
     command.  There are two other ways to do it, however: one is to use the
     <command>FOR-IN-EXECUTE</>
     loop form described in <xref linkend="plpgsql-records-iterating">,
     and the other is to use a cursor with <command>OPEN-FOR-EXECUTE</>, as
     described in <xref linkend="plpgsql-cursor-opening">.
    </para>

    <para>
     When working with dynamic commands you will often have to handle escaping
     of single quotes.  The recommended method for quoting fixed text in your
     function body is dollar quoting.  (If you have legacy code that does
     not use dollar quoting, please refer to the
     overview in <xref linkend="plpgsql-quote-tips">, which can save you
     some effort when translating said code to a more reasonable scheme.)
    </para>

    <para>
     Dynamic values that are to be inserted into the constructed
     query require special handling since they might themselves contain
     quote characters.
     An example (this assumes that you are using dollar quoting for the
     function as a whole, so the quote marks need not be doubled):
<programlisting>
EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = '
        || quote_literal(newvalue)
        || ' WHERE key = '
        || quote_literal(keyvalue);
</programlisting>
    </para>

    <para>
     This example shows use of the functions
     <function>quote_ident(<type>text</type>)</function> and
     <function>quote_literal(<type>text</type>)</function>.<indexterm><primary>quote_ident</><secondary>use
     in
     PL/pgSQL</></indexterm><indexterm><primary>quote_literal</><secondary>use
     in PL/pgSQL</></indexterm> For safety, variables containing column and
     table identifiers should be passed to function
     <function>quote_ident</function>.  Variables containing values
     that should be literal strings in the constructed command should
     be passed to <function>quote_literal</function>.  Both take the
     appropriate steps to return the input text enclosed in double or
     single quotes respectively, with any embedded special characters
     properly escaped.
    </para>

    <para>
     Note that dollar quoting is only useful for quoting fixed text.
     It would be a very bad idea to try to do the above example as
<programlisting>
EXECUTE 'UPDATE tbl SET '
        || quote_ident(colname)
        || ' = $$'
        || newvalue
        || '$$ WHERE key = '
        || quote_literal(keyvalue);
</programlisting>
     because it would break if the contents of <literal>newvalue</>
     happened to contain <literal>$$</>.  The same objection would
     apply to any other dollar-quoting delimiter you might pick.
     So, to safely quote text that is not known in advance, you
     <emphasis>must</> use <function>quote_literal</function>.
    </para>

    <para>
     A much larger example of a dynamic command and
     <command>EXECUTE</command> can be seen in <xref
     linkend="plpgsql-porting-ex2">, which builds and executes a
     <command>CREATE FUNCTION</> command to define a new function.
    </para>
   </sect2>

   <sect2 id="plpgsql-statements-diagnostics">
    <title>Obtaining the Result Status</title>

    <para>
     There are several ways to determine the effect of a command. The
     first method is to use the <command>GET DIAGNOSTICS</command>
     command, which has the form:

<synopsis>
GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional> ;
</synopsis>

     This command allows retrieval of system status indicators.  Each
     <replaceable>item</replaceable> is a key word identifying a state
     value to be assigned to the specified variable (which should be
     of the right data type to receive it).  The currently available
     status items are <varname>ROW_COUNT</>, the number of rows
     processed by the last <acronym>SQL</acronym> command sent down to
     the <acronym>SQL</acronym> engine, and <varname>RESULT_OID</>,
     the OID of the last row inserted by the most recent
     <acronym>SQL</acronym> command.  Note that <varname>RESULT_OID</>
     is only useful after an <command>INSERT</command> command.
    </para>

    <para>
     An example:
<programlisting>
GET DIAGNOSTICS integer_var = ROW_COUNT;
</programlisting>
    </para>

    <para>
     The second method to determine the effects of a command is to check the
     special variable named <literal>FOUND</literal>, which is of
     type <type>boolean</type>.  <literal>FOUND</literal> starts out
     false within each <application>PL/pgSQL</application> function call.
     It is set by each of the following types of statements:
         <itemizedlist>
          <listitem>
           <para>
                A <command>SELECT INTO</command> statement sets
                <literal>FOUND</literal> true if it returns a row, false if no
                row is returned.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>PERFORM</> statement sets <literal>FOUND</literal>
                true if it produces (and discards) a row, false if no row is
                produced.
           </para>
          </listitem>
          <listitem>
           <para>
                <command>UPDATE</>, <command>INSERT</>, and <command>DELETE</>
                statements set <literal>FOUND</literal> true if at least one
                row is affected, false if no row is affected.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>FETCH</> statement sets <literal>FOUND</literal>
                true if it returns a row, false if no row is returned.
           </para>
          </listitem>
          <listitem>
           <para>
                A <command>FOR</> statement sets <literal>FOUND</literal> true
                if it iterates one or more times, else false.  This applies to
                all three variants of the <command>FOR</> statement (integer
                <command>FOR</> loops, record-set <command>FOR</> loops, and
                dynamic record-set <command>FOR</>
                loops). <literal>FOUND</literal> is set this way when the
                <command>FOR</> loop exits; inside the execution of the loop,
                <literal>FOUND</literal> is not modified by the
                <command>FOR</> statement, although it may be changed by the
                execution of other statements within the loop body.
           </para>
          </listitem>
         </itemizedlist>

     <literal>FOUND</literal> is a local variable within each
     <application>PL/pgSQL</application> function; so any changes
     to it affect only the current function.
    </para>

   </sect2>
  </sect1>

  <sect1 id="plpgsql-control-structures">
   <title>Control Structures</title>

   <para>
    Control structures are probably the most useful (and
    important) part of <application>PL/pgSQL</>. With
    <application>PL/pgSQL</>'s control structures,
    you can manipulate <productname>PostgreSQL</> data in a very
    flexible and powerful way. 
   </para>

   <sect2 id="plpgsql-statements-returning">
    <title>Returning From a Function</title>

    <para>
     There are two commands available that allow you to return data
     from a function: <command>RETURN</command> and <command>RETURN
     NEXT</command>.
    </para>

    <sect3>
     <title><command>RETURN</></title>

<synopsis>
RETURN <replaceable>expression</replaceable>;
</synopsis>

     <para>
      <command>RETURN</command> with an expression terminates the
      function and returns the value of
      <replaceable>expression</replaceable> to the caller.  This form
      is to be used for <application>PL/pgSQL</> functions that do
      not return a set.
     </para>

     <para>
      When returning a scalar type, any expression can be used. The
      expression's result will be automatically cast into the
      function's return type as described for assignments. To return a
      composite (row) value, you must write a record or row variable
      as the <replaceable>expression</replaceable>.
     </para>

     <para>
      The return value of a function cannot be left undefined. If
      control reaches the end of the top-level block of the function
      without hitting a <command>RETURN</command> statement, a run-time
      error will occur.
     </para>

     <para>
      If you have declared the function to
      return <type>void</type>, a <command>RETURN</command> statement
      must still be provided; but in this case the expression following
      <command>RETURN</command> is optional and will be ignored if present.
     </para>
    </sect3>

    <sect3>
     <title><command>RETURN NEXT</></title>

<synopsis>
RETURN NEXT <replaceable>expression</replaceable>;
</synopsis>

     <para>
      When a <application>PL/pgSQL</> function is declared to return
      <literal>SETOF <replaceable>sometype</></literal>, the procedure
      to follow is slightly different.  In that case, the individual
      items to return are specified in <command>RETURN NEXT</command>
      commands, and then a final <command>RETURN</command> command
      with no argument is used to indicate that the function has
      finished executing.  <command>RETURN NEXT</command> can be used
      with both scalar and composite data types; in the latter case, an
      entire <quote>table</quote> of results will be returned.
     </para>

     <para>
      Functions that use <command>RETURN NEXT</command> should be
      called in the following fashion:

<programlisting>
SELECT * FROM some_func();
</programlisting>

      That is, the function must be used as a table source in a
      <literal>FROM</literal> clause.
     </para>

     <para>
      <command>RETURN NEXT</command> does not actually return from the
      function; it simply saves away the value of the expression.
      Execution then continues with the next statement in
      the <application>PL/pgSQL</> function.  As successive
      <command>RETURN NEXT</command> commands are executed, the result
      set is built up.  A final <command>RETURN</command>, which should
      have no argument, causes control to exit the function.
     </para>

     <note>
      <para>
       The current implementation of <command>RETURN NEXT</command>
       for <application>PL/pgSQL</> stores the entire result set
       before returning from the function, as discussed above.  That
       means that if a <application>PL/pgSQL</> function produces a
       very large result set, performance may be poor: data will be
       written to disk to avoid memory exhaustion, but the function
       itself will not return until the entire result set has been
       generated.  A future version of <application>PL/pgSQL</> may
       allow users to define set-returning functions
       that do not have this limitation.  Currently, the point at
       which data begins being written to disk is controlled by the
       <varname>work_mem</> configuration variable.  Administrators
       who have sufficient memory to store larger result sets in
       memory should consider increasing this parameter.
      </para>
     </note>
    </sect3>
   </sect2>

   <sect2 id="plpgsql-conditionals">
    <title>Conditionals</title>

    <para>
     <literal>IF</> statements let you execute commands based on
     certain conditions.  <application>PL/pgSQL</> has five forms of
     <literal>IF</>:
    <itemizedlist>
     <listitem>
      <para><literal>IF ... THEN</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSE</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSE IF</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSIF ... THEN ... ELSE</></>
     </listitem>
     <listitem>
      <para><literal>IF ... THEN ... ELSEIF ... THEN ... ELSE</></>
     </listitem>
    </itemizedlist>
    </para>

    <sect3>
     <title><literal>IF-THEN</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN</literal> statements are the simplest form of
        <literal>IF</literal>. The statements between
        <literal>THEN</literal> and <literal>END IF</literal> will be
        executed if the condition is true. Otherwise, they are
        skipped.
       </para>

       <para>
        Example:
<programlisting>
IF v_user_id &lt;&gt; 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSE</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
ELSE
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN-ELSE</literal> statements add to
        <literal>IF-THEN</literal> by letting you specify an
        alternative set of statements that should be executed if the
        condition evaluates to false.
       </para>

       <para>
        Examples:
<programlisting>
IF parentid IS NULL OR parentid = ''
THEN
    RETURN fullname;
ELSE
    RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
</programlisting>

<programlisting>
IF v_count &gt; 0 THEN 
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;
</programlisting>
     </para>
    </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSE IF</></title>

       <para>
        <literal>IF</literal> statements can be nested, as in the
        following example:

<programlisting>
IF demo_row.sex = 'm' THEN
    pretty_sex := 'man';
ELSE
    IF demo_row.sex = 'f' THEN
        pretty_sex := 'woman';
    END IF;
END IF;
</programlisting>
       </para>

       <para>
        When you use this form, you are actually nesting an
        <literal>IF</literal> statement inside the
        <literal>ELSE</literal> part of an outer <literal>IF</literal>
        statement. Thus you need one <literal>END IF</literal>
        statement for each nested <literal>IF</literal> and one for the parent
        <literal>IF-ELSE</literal>.  This is workable but grows
        tedious when there are many alternatives to be checked.
        Hence the next form.
       </para>
     </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSIF-ELSE</></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
    ...
</optional>
</optional>
<optional> ELSE
    <replaceable>statements</replaceable> </optional>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN-ELSIF-ELSE</> provides a more convenient
        method of checking many alternatives in one statement.
        Formally it is equivalent to nested
        <literal>IF-THEN-ELSE-IF-THEN</> commands, but only one
        <literal>END IF</> is needed.
       </para>

       <para>
        Here is an example:

<programlisting>
IF number = 0 THEN
    result := 'zero';
ELSIF number &gt; 0 THEN 
    result := 'positive';
ELSIF number &lt; 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>IF-THEN-ELSEIF-ELSE</></title>

      <para>
       <literal>ELSEIF</> is an alias for <literal>ELSIF</>.
      </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-control-structures-loops">
    <title>Simple Loops</title>

    <indexterm zone="plpgsql-control-structures-loops">
     <primary>loop</primary>
     <secondary>in PL/pgSQL</secondary>
    </indexterm>

    <para>
     With the <literal>LOOP</>, <literal>EXIT</>, <literal>WHILE</>,
     and <literal>FOR</> statements, you can arrange for your
     <application>PL/pgSQL</application> function to repeat a series
     of commands.
    </para>

    <sect3>
     <title><literal>LOOP</></title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

     <para>
      <literal>LOOP</> defines an unconditional loop that is repeated indefinitely
      until terminated by an <literal>EXIT</> or <command>RETURN</command>
      statement.  The optional label can be used by <literal>EXIT</> statements in
      nested loops to specify which level of nesting should be
      terminated.
     </para>
    </sect3>

     <sect3>
      <title><literal>EXIT</></title>

<synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>;
</synopsis>

       <para>
        If no <replaceable>label</replaceable> is given,
        the innermost loop is terminated and the
        statement following <literal>END LOOP</> is executed next.
        If <replaceable>label</replaceable> is given, it
        must be the label of the current or some outer level of nested loop
        or block. Then the named loop or block is terminated and control
        continues with the statement after the loop's/block's corresponding
        <literal>END</>.
       </para>

       <para>
        If <literal>WHEN</> is present, loop exit occurs only if the specified
        condition is true, otherwise control passes to the statement after
        <literal>EXIT</>.
       </para>

       <para>
        <literal>EXIT</> can be used to cause early exit from all types of
        loops; it is not limited to use with unconditional loops.
       </para>

       <para>
        Examples:
<programlisting>
LOOP
    -- some computations
    IF count &gt; 0 THEN
        EXIT;  -- exit loop
    END IF;
END LOOP;

LOOP
    -- some computations
    EXIT WHEN count &gt; 0;  -- same result as previous example
END LOOP;

BEGIN
    -- some computations
    IF stocks &gt; 100000 THEN
        EXIT;  -- causes exit from the BEGIN block
    END IF;
END;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>WHILE</></title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
WHILE <replaceable>expression</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

       <para>
        The <literal>WHILE</> statement repeats a
        sequence of statements so long as the condition expression
        evaluates to true.  The condition is checked just before
        each entry to the loop body.
       </para>

       <para>
        For example:
<programlisting>
WHILE amount_owed &gt; 0 AND gift_certificate_balance &gt; 0 LOOP
    -- some computations here
END LOOP;

WHILE NOT boolean_expression LOOP
    -- some computations here
END LOOP;
</programlisting>
       </para>
     </sect3>

     <sect3>
      <title><literal>FOR</> (integer variant)</title>

<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>

       <para>
        This form of <literal>FOR</> creates a loop that iterates over a range of integer
        values. The variable 
        <replaceable>name</replaceable> is automatically defined as type
        <type>integer</> and exists only inside the loop. The two expressions giving
        the lower and upper bound of the range are evaluated once when entering
        the loop. The iteration step is normally 1, but is -1 when <literal>REVERSE</> is
        specified.
       </para>

       <para>
        Some examples of integer <literal>FOR</> loops:
<programlisting>
FOR i IN 1..10 LOOP
    -- some computations here
    RAISE NOTICE 'i is %', i;
END LOOP;

FOR i IN REVERSE 10..1 LOOP
    -- some computations here
END LOOP;
</programlisting>
       </para>

       <para>
        If the lower bound is greater than the upper bound (or less than,
        in the <literal>REVERSE</> case), the loop body is not
        executed at all.  No error is raised.
       </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-records-iterating">
    <title>Looping Through Query Results</title>

    <para>
     Using a different type of <literal>FOR</> loop, you can iterate through
     the results of a query and manipulate that data
     accordingly. The syntax is:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN <replaceable>query</replaceable> LOOP
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
     The record or row variable is successively assigned each row
     resulting from the <replaceable>query</replaceable> (which must be a
     <command>SELECT</command> command) and the loop body is executed for each
     row. Here is an example:
<programlisting>
CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS $$
DECLARE
    mviews RECORD;
BEGIN
    PERFORM cs_log('Refreshing materialized views...');

    FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP

        -- Now "mviews" has one record from cs_materialized_views

        PERFORM cs_log('Refreshing materialized view ' || quote_ident(mviews.mv_name) || ' ...');
        EXECUTE 'TRUNCATE TABLE ' || quote_ident(mviews.mv_name);
        EXECUTE 'INSERT INTO ' || quote_ident(mviews.mv_name) || ' ' || mviews.mv_query;
    END LOOP;

    PERFORM cs_log('Done refreshing materialized views.');
    RETURN 1;
END;
$$ LANGUAGE plpgsql;
</programlisting>

     If the loop is terminated by an <literal>EXIT</> statement, the last
     assigned row value is still accessible after the loop.
    </para>

    <para>
     The <literal>FOR-IN-EXECUTE</> statement is another way to iterate over
     rows:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> LOOP 
    <replaceable>statements</replaceable>
END LOOP;
</synopsis>
     This is like the previous form, except that the source
     <command>SELECT</command> statement is specified as a string
     expression, which is evaluated and replanned on each entry to
     the <literal>FOR</> loop.  This allows the programmer to choose the speed of
     a preplanned query or the flexibility of a dynamic query, just
     as with a plain <command>EXECUTE</command> statement.
    </para>

    <note>
    <para>
     The <application>PL/pgSQL</> parser presently distinguishes the
     two kinds of <literal>FOR</> loops (integer or query result) by checking
     whether <literal>..</> appears outside any parentheses between
     <literal>IN</> and <literal>LOOP</>.  If <literal>..</> is not seen then
     the loop is presumed to be a loop over rows.  Mistyping the <literal>..</>
     is thus likely to lead to a complaint along the lines of
     <quote>loop variable of loop over rows must be a record or row variable</>,
     rather than the simple syntax error one might expect to get.
    </para>
    </note>
   </sect2>

   <sect2 id="plpgsql-error-trapping">
    <title>Trapping Errors</title>

    <para>
     By default, any error occurring in a <application>PL/pgSQL</>
     function aborts execution of the function, and indeed of the
     surrounding transaction as well.  You can trap errors and recover
     from them by using a <command>BEGIN</> block with an
     <literal>EXCEPTION</> clause.  The syntax is an extension of the
     normal syntax for a <command>BEGIN</> block:

<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
    <replaceable>declarations</replaceable> </optional>
BEGIN
    <replaceable>statements</replaceable>
EXCEPTION
    WHEN <replaceable>condition</replaceable> <optional> OR <replaceable>condition</replaceable> ... </optional> THEN
        <replaceable>handler_statements</replaceable>
    <optional> WHEN <replaceable>condition</replaceable> <optional> OR <replaceable>condition</replaceable> ... </optional> THEN
          <replaceable>handler_statements</replaceable>
      ... </optional>
END;
</synopsis>
    </para>

    <para>
     If no error occurs, this form of block simply executes all the
     <replaceable>statements</replaceable>, and then control passes
     to the next statement after <literal>END</>.  But if an error
     occurs within the <replaceable>statements</replaceable>, further
     processing of the <replaceable>statements</replaceable> is
     abandoned, and control passes to the <literal>EXCEPTION</> list.
     The list is searched for the first <replaceable>condition</replaceable>
     matching the error that occurred.  If a match is found, the
     corresponding <replaceable>handler_statements</replaceable> are
     executed, and then control passes to the next statement after
     <literal>END</>.  If no match is found, the error propagates out
     as though the <literal>EXCEPTION</> clause were not there at all:
     the error can be caught by an enclosing block with
     <literal>EXCEPTION</>, or if there is none it aborts processing
     of the function.
    </para>

    <para>
     The <replaceable>condition</replaceable> names can be any of those
     shown in <xref linkend="errcodes-appendix">.  A category name matches
     any error within its category.
     The special condition name <literal>OTHERS</>
     matches every error type except <literal>QUERY_CANCELED</>.
     (It is possible, but often unwise, to trap
     <literal>QUERY_CANCELED</> by name.)
     Condition names are not case-sensitive.
    </para>

    <para>
     If a new error occurs within the selected
     <replaceable>handler_statements</replaceable>, it cannot be caught
     by this <literal>EXCEPTION</> clause, but is propagated out.
     A surrounding <literal>EXCEPTION</> clause could catch it.
    </para>

    <para>
     When an error is caught by an <literal>EXCEPTION</> clause,
     the local variables of the <application>PL/pgSQL</> function
     remain as they were when the error occurred, but all changes
     to persistent database state within the block are rolled back.
     As an example, consider this fragment:

<programlisting>
    INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
    BEGIN
        UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
        x := x + 1;
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            RAISE NOTICE 'caught division_by_zero';
            RETURN x;
    END;
</programlisting>

     When control reaches the assignment to <literal>y</>, it will
     fail with a <literal>division_by_zero</> error.  This will be caught by
     the <literal>EXCEPTION</> clause.  The value returned in the
     <command>RETURN</> statement will be the incremented value of
     <literal>x</>, but the effects of the <command>UPDATE</> command will
     have been rolled back.  The <command>INSERT</> command preceding the
     block is not rolled back, however, so the end result is that the database
     contains <literal>Tom Jones</> not <literal>Joe Jones</>.
    </para>

    <tip>
     <para>
      A block containing an <literal>EXCEPTION</> clause is significantly
      more expensive to enter and exit than a block without one.  Therefore,
      don't use <literal>EXCEPTION</> without need.
     </para>
    </tip>

    <example id="plpgsql-upsert-example">
    <title>Exceptions with <command>UPDATE</>/<command>INSERT</></title>
    <para>


    This example uses exception handling to perform either
    <command>UPDATE</> or <command>INSERT</>, as appropriate.

<programlisting>
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
    LOOP
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;

        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- do nothing
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
</programlisting>

    </para>
    </example>
  </sect2>
  </sect1>

  <sect1 id="plpgsql-cursors">
   <title>Cursors</title>

   <indexterm zone="plpgsql-cursors">
    <primary>cursor</primary>
    <secondary>in PL/pgSQL</secondary>
   </indexterm>

   <para>
    Rather than executing a whole query at once, it is possible to set
    up a <firstterm>cursor</> that encapsulates the query, and then read
    the query result a few rows at a time. One reason for doing this is
    to avoid memory overrun when the result contains a large number of
    rows. (However, <application>PL/pgSQL</> users do not normally need
    to worry about that, since <literal>FOR</> loops automatically use a cursor
    internally to avoid memory problems.) A more interesting usage is to
    return a reference to a cursor that a function has created, allowing the
    caller to read the rows. This provides an efficient way to return
    large row sets from functions.
   </para>

   <sect2 id="plpgsql-cursor-declarations">
    <title>Declaring Cursor Variables</title>

    <para>
     All access to cursors in <application>PL/pgSQL</> goes through
     cursor variables, which are always of the special data type
     <type>refcursor</>.  One way to create a cursor variable
     is just to declare it as a variable of type <type>refcursor</>.
     Another way is to use the cursor declaration syntax,
     which in general is:
<synopsis>
<replaceable>name</replaceable> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>query</replaceable> ;
</synopsis>
     (<literal>FOR</> may be replaced by <literal>IS</> for
     <productname>Oracle</productname> compatibility.)
     <replaceable>arguments</replaceable>, if specified, is a
     comma-separated list of pairs <literal><replaceable>name</replaceable>
     <replaceable>datatype</replaceable></literal> that define names to be
     replaced by parameter values in the given query.  The actual
     values to substitute for these names will be specified later,
     when the cursor is opened.
    </para>
    <para>
     Some examples:
<programlisting>
DECLARE
    curs1 refcursor;
    curs2 CURSOR FOR SELECT * FROM tenk1;
    curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;
</programlisting>
     All three of these variables have the data type <type>refcursor</>,
     but the first may be used with any query, while the second has
     a fully specified query already <firstterm>bound</> to it, and the last
     has a parameterized query bound to it.  (<literal>key</> will be
     replaced by an integer parameter value when the cursor is opened.)
     The variable <literal>curs1</>
     is said to be <firstterm>unbound</> since it is not bound to
     any particular query.
    </para>
   </sect2>

   <sect2 id="plpgsql-cursor-opening">
    <title>Opening Cursors</title>

    <para>
     Before a cursor can be used to retrieve rows, it must be
     <firstterm>opened</>. (This is the equivalent action to the SQL
     command <command>DECLARE CURSOR</>.) <application>PL/pgSQL</> has
     three forms of the <command>OPEN</> statement, two of which use unbound
     cursor variables while the third uses a bound cursor variable.
    </para>

    <sect3>
     <title><command>OPEN FOR SELECT</command></title>

<synopsis>
OPEN <replaceable>unbound_cursor</replaceable> FOR SELECT ...;
</synopsis>

       <para>
    The cursor variable is opened and given the specified query to
        execute.  The cursor cannot be open already, and it must have been
        declared as an unbound cursor (that is, as a simple
        <type>refcursor</> variable).  The <command>SELECT</command> query
        is treated in the same way as other <command>SELECT</command>
        statements in <application>PL/pgSQL</>: <application>PL/pgSQL</>
        variable names are substituted, and the query plan is cached for
        possible reuse.
       </para>

       <para>
        An example:
<programlisting>
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title><command>OPEN FOR EXECUTE</command></title>

<synopsis>
OPEN <replaceable>unbound_cursor</replaceable> FOR EXECUTE <replaceable class="command">query_string</replaceable>;
</synopsis>

         <para>
          The cursor variable is opened and given the specified query to
          execute.  The cursor cannot be open already, and it must have been
          declared as an unbound cursor (that is, as a simple
          <type>refcursor</> variable).  The query is specified as a string
          expression in the same way as in the <command>EXECUTE</command>
          command.  As usual, this gives flexibility so the query can vary
          from one run to the next.
       </para>

       <para>
        An example:
<programlisting>
OPEN curs1 FOR EXECUTE 'SELECT * FROM ' || quote_ident($1);
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title>Opening a Bound Cursor</title>

<synopsis>
OPEN <replaceable>bound_cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional>;
</synopsis>

         <para>
          This form of <command>OPEN</command> is used to open a cursor
          variable whose query was bound to it when it was declared.  The
          cursor cannot be open already.  A list of actual argument value
          expressions must appear if and only if the cursor was declared to
          take arguments.  These values will be substituted in the query.
          The query plan for a bound cursor is always considered cacheable;
          there is no equivalent of <command>EXECUTE</command> in this case.
         </para>

    <para>
     Examples:
<programlisting>
OPEN curs2;
OPEN curs3(42);
</programlisting>
       </para>
     </sect3>
   </sect2>

   <sect2 id="plpgsql-cursor-using">
    <title>Using Cursors</title>

    <para>
     Once a cursor has been opened, it can be manipulated with the
     statements described here.
    </para>

    <para>
     These manipulations need not occur in the same function that
     opened the cursor to begin with.  You can return a <type>refcursor</>
     value out of a function and let the caller operate on the cursor.
     (Internally, a <type>refcursor</> value is simply the string name
     of a so-called portal containing the active query for the cursor.  This name
     can be passed around, assigned to other <type>refcursor</> variables,
     and so on, without disturbing the portal.)
    </para>

    <para>
     All portals are implicitly closed at transaction end.  Therefore
     a <type>refcursor</> value is usable to reference an open cursor
     only until the end of the transaction.
    </para>

    <sect3>
     <title><literal>FETCH</></title>

<synopsis>
FETCH <replaceable>cursor</replaceable> INTO <replaceable>target</replaceable>;
</synopsis>

         <para>
          <command>FETCH</command> retrieves the next row from the
          cursor into a target, which may be a row variable, a record
          variable, or a comma-separated list of simple variables, just like
          <command>SELECT INTO</command>.  As with <command>SELECT
           INTO</command>, the special variable <literal>FOUND</literal> may
          be checked to see whether a row was obtained or not.
         </para>

    <para>
     An example:
<programlisting>
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title><literal>CLOSE</></title>

<synopsis>
CLOSE <replaceable>cursor</replaceable>;
</synopsis>

       <para>
        <command>CLOSE</command> closes the portal underlying an open
        cursor.  This can be used to release resources earlier than end of
        transaction, or to free up the cursor variable to be opened again.
       </para>

       <para>
        An example:
<programlisting>
CLOSE curs1;
</programlisting>
       </para>
     </sect3>

    <sect3>
     <title>Returning Cursors</title>

       <para>
        <application>PL/pgSQL</> functions can return cursors to the
        caller. This is useful to return multiple rows or columns,
        especially with very large result sets.  To do this, the function
        opens the cursor and returns the cursor name to the caller (or simply
        opens the cursor using a portal name specified by or otherwise known
        to the caller).  The caller can then fetch rows from the cursor. The
        cursor can be closed by the caller, or it will be closed automatically
        when the transaction closes.
       </para>

       <para>
        The portal name used for a cursor can be specified by the
        programmer or automatically generated.  To specify a portal name,
        simply assign a string to the <type>refcursor</> variable before
        opening it.  The string value of the <type>refcursor</> variable
        will be used by <command>OPEN</> as the name of the underlying portal.
        However, if the <type>refcursor</> variable is null,
        <command>OPEN</> automatically generates a name that does not
        conflict with any existing portal, and assigns it to the
        <type>refcursor</> variable.
       </para>

       <note>
        <para>
         A bound cursor variable is initialized to the string value
         representing its name, so that the portal name is the same as
         the cursor variable name, unless the programmer overrides it
         by assignment before opening the cursor.  But an unbound cursor
         variable defaults to the null value initially , so it will receive
         an automatically-generated unique name, unless overridden.
        </para>
       </note>

       <para>
        The following example shows one way a cursor name can be supplied by
        the caller:

<programlisting>
CREATE TABLE test (col text);
INSERT INTO test VALUES ('123');

CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
BEGIN
    OPEN $1 FOR SELECT col FROM test;
    RETURN $1;
END;
' LANGUAGE plpgsql;

BEGIN;
SELECT reffunc('funccursor');
FETCH ALL IN funccursor;
COMMIT;
</programlisting>
       </para>

       <para>
        The following example uses automatic cursor name generation:

<programlisting>
CREATE FUNCTION reffunc2() RETURNS refcursor AS '
DECLARE
    ref refcursor;
BEGIN
    OPEN ref FOR SELECT col FROM test;
    RETURN ref;
END;
' LANGUAGE plpgsql;

BEGIN;
SELECT reffunc2();

      reffunc2      
--------------------
 &lt;unnamed cursor 1&gt;
(1 row)

FETCH ALL IN "&lt;unnamed cursor 1&gt;";
COMMIT;
</programlisting>
       </para>

       <para>
        The following example shows one way to return multiple cursors
        from a single function:

<programlisting>
CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
BEGIN
    OPEN $1 FOR SELECT * FROM table_1;
    RETURN NEXT $1;
    OPEN $2 FOR SELECT * FROM table_2;
    RETURN NEXT $2;
    RETURN;
END;
$$ LANGUAGE plpgsql;

-- need to be in a transaction to use cursors.
BEGIN;

SELECT * FROM myfunc('a', 'b');

FETCH ALL FROM a;
FETCH ALL FROM b;
COMMIT;
</programlisting>
       </para>
     </sect3>
   </sect2>
  </sect1>

  <sect1 id="plpgsql-errors-and-messages">
   <title>Errors and Messages</title>

   <para>
    Use the <command>RAISE</command> statement to report messages and
    raise errors.

<synopsis>
RAISE <replaceable class="parameter">level</replaceable> '<replaceable class="parameter">format</replaceable>' <optional>, <replaceable class="parameter">variable</replaceable> <optional>, ...</optional></optional>;
</synopsis>

    Possible levels are <literal>DEBUG</literal>,
    <literal>LOG</literal>, <literal>INFO</literal>,
    <literal>NOTICE</literal>, <literal>WARNING</literal>,
    and <literal>EXCEPTION</literal>.
    <literal>EXCEPTION</literal> raises an error (which normally aborts the
    current transaction); the other levels only generate messages of different
    priority levels.
    Whether messages of a particular priority are reported to the client,
    written to the server log, or both is controlled by the
    <xref linkend="guc-log-min-messages"> and
    <xref linkend="guc-client-min-messages"> configuration
    variables. See <xref linkend="runtime-config"> for more
    information.
   </para>

   <para>
    Inside the format string, <literal>%</literal> is replaced by the
    next optional argument's string representation. Write
    <literal>%%</literal> to emit a literal <literal>%</literal>. Note
    that the optional arguments must presently be simple variables,
    not expressions, and the format must be a simple string literal.
   </para>

   <!--
   This example should work, but does not:
        RAISE NOTICE 'Id number ' || key || ' not found!';
   Put it back when we allow non-string-literal formats.
    -->

   <para>
    In this example, the value of <literal>v_job_id</> will replace the
    <literal>%</literal> in the string:
<programlisting>
RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;
</programlisting>
   </para>

   <para>
    This example will abort the transaction with the given error message:
<programlisting>
RAISE EXCEPTION 'Nonexistent ID --> %', user_id;
</programlisting>
   </para>

    <para>
     <command>RAISE EXCEPTION</command> presently always generates
     the same SQLSTATE code, <literal>P0001</>, no matter what message
     it is invoked with.  It is possible to trap this exception with
     <literal>EXCEPTION ... WHEN RAISE_EXCEPTION THEN ...</> but there
     is no way to tell one <command>RAISE</> from another.
    </para>
 </sect1>

 <sect1 id="plpgsql-trigger">
  <title>Trigger Procedures</title>

  <indexterm zone="plpgsql-trigger">
   <primary>trigger</primary>
   <secondary>in PL/pgSQL</secondary>
  </indexterm>

  <para>
        <application>PL/pgSQL</application> can be used to define trigger
        procedures. A trigger procedure is created with the
        <command>CREATE FUNCTION</> command, declaring it as a function with
        no arguments and a return type of <type>trigger</type>.  Note that
        the function must be declared with no arguments even if it expects
        to receive arguments specified in <command>CREATE TRIGGER</> &mdash;
        trigger arguments are passed via <varname>TG_ARGV</>, as described
        below.
  </para>

  <para>
   When a <application>PL/pgSQL</application> function is called as a
   trigger, several special variables are created automatically in the 
   top-level block. They are:

   <variablelist>
    <varlistentry>
     <term><varname>NEW</varname></term>
     <listitem>
      <para>
       Data type <type>RECORD</type>; variable holding the new
       database row for <command>INSERT</>/<command>UPDATE</> operations in row-level
       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>OLD</varname></term>
     <listitem>
      <para>
       Data type <type>RECORD</type>; variable holding the old
       database row for <command>UPDATE</>/<command>DELETE</> operations in row-level
       triggers. This variable is <symbol>NULL</symbol> in statement-level triggers.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_NAME</varname></term>
     <listitem>
      <para>
       Data type <type>name</type>; variable that contains the name of the trigger actually
       fired.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_WHEN</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of either 
              <literal>BEFORE</literal> or <literal>AFTER</literal>
              depending on the trigger's definition.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_LEVEL</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of either
       <literal>ROW</literal> or <literal>STATEMENT</literal>
       depending on the trigger's definition.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_OP</varname></term>
     <listitem>
      <para>
       Data type <type>text</type>; a string of
       <literal>INSERT</literal>, <literal>UPDATE</literal>, or
       <literal>DELETE</literal> telling for which operation the
       trigger was fired.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_RELID</varname></term>
     <listitem>
      <para>
       Data type <type>oid</type>; the object ID of the table that caused the
       trigger invocation.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_RELNAME</varname></term>
     <listitem>
      <para>
       Data type <type>name</type>; the name of the table that caused the trigger
       invocation.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_NARGS</varname></term>
     <listitem>
      <para>
       Data type <type>integer</type>; the number of arguments given to the trigger
       procedure in the <command>CREATE TRIGGER</command> statement.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><varname>TG_ARGV[]</varname></term>
     <listitem>
      <para>
       Data type array of <type>text</type>; the arguments from
              the <command>CREATE TRIGGER</command> statement.
       The index counts from 0. Invalid
       indices (less than 0 or greater than or equal to <varname>tg_nargs</>) result in a null value.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

   <para>
    A trigger function must return either <symbol>NULL</symbol> or a
    record/row value having exactly the structure of the table the
    trigger was fired for.
   </para>

   <para>
    Row-level triggers fired <literal>BEFORE</> may return null to signal the
    trigger manager to skip the rest of the operation for this row
    (i.e., subsequent triggers are not fired, and the
    <command>INSERT</>/<command>UPDATE</>/<command>DELETE</> does not occur
    for this row).  If a nonnull 
    value is returned then the operation proceeds with that row value.
    Returning a row value different from the original value
    of <varname>NEW</> alters the row that will be inserted or updated
    (but has no direct effect in the <command>DELETE</> case).
    To alter the row to be stored, it is possible to replace single values
    directly in <varname>NEW</> and return the modified <varname>NEW</>,
    or to build a complete new record/row to return.
   </para>

   <para>
    The return value of a <literal>BEFORE</> or <literal>AFTER</>
    statement-level trigger or an <literal>AFTER</> row-level trigger is
    always ignored; it may as well be null. However, any of these types of
    triggers can still abort the entire operation by raising an error.
   </para>

   <para>
    <xref linkend="plpgsql-trigger-example"> shows an example of a
    trigger procedure in <application>PL/pgSQL</application>.
   </para>

   <example id="plpgsql-trigger-example">
    <title>A <application>PL/pgSQL</application> Trigger Procedure</title>

    <para>
     This example trigger ensures that any time a row is inserted or updated
     in the table, the current user name and time are stamped into the
     row. And it checks that an employee's name is given and that the
     salary is a positive value.
    </para>

<programlisting>
CREATE TABLE emp (
    empname text,
    salary integer,
    last_date timestamp,
    last_user text
);

CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
    BEGIN
        -- Check that empname and salary are given
        IF NEW.empname IS NULL THEN
            RAISE EXCEPTION 'empname cannot be null';
        END IF;
        IF NEW.salary IS NULL THEN
            RAISE EXCEPTION '% cannot have null salary', NEW.empname;
        END IF;

        -- Who works for us when she must pay for it?
        IF NEW.salary &lt; 0 THEN
            RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
        END IF;

        -- Remember who changed the payroll when
        NEW.last_date := 'now';
        NEW.last_user := current_user;
        RETURN NEW;
    END;
$emp_stamp$ LANGUAGE plpgsql;

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
    FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
</programlisting>
   </example>

   <para>
    Another way to log changes to a table involves creating a new table that
    holds a row for each insert, update, or delete that occurs. This approach
    can be thought of as auditing changes to a table.
    <xref linkend="plpgsql-trigger-audit-example"> shows an example of an
    audit trigger procedure in <application>PL/pgSQL</application>.
   </para>

   <example id="plpgsql-trigger-audit-example">
    <title>A <application>PL/pgSQL</application> Trigger Procedure For Auditing</title>

    <para>
     This example trigger ensures that any insert, update or delete of a row
     in the <literal>emp</literal> table is recorded (i.e., audited) in the <literal>emp_audit</literal> table. 
     The current time and user name are stamped into the row, together with 
     the type of operation performed on it.
    </para>

<programlisting>
CREATE TABLE emp (
    empname           text NOT NULL,
    salary            integer
);

CREATE TABLE emp_audit( 
    operation         char(1)   NOT NULL,
    stamp             timestamp NOT NULL,
    userid            text      NOT NULL,
    empname           text      NOT NULL,
    salary integer
);

CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
    BEGIN
        --
        -- Create a row in emp_audit to reflect the operation performed on emp,
        -- make use of the special variable TG_OP to work out the operation.
        --
        IF (TG_OP = 'DELETE') THEN
            INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*;
            RETURN OLD;
        ELSIF (TG_OP = 'UPDATE') THEN
            INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
            RETURN NEW;
        ELSIF (TG_OP = 'INSERT') THEN
            INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*;
            RETURN NEW;
        END IF;
        RETURN NULL; -- result is ignored since this is an AFTER trigger
    END;
$emp_audit$ language plpgsql;

CREATE TRIGGER emp_audit
AFTER INSERT OR UPDATE OR DELETE ON emp
    FOR EACH ROW EXECUTE PROCEDURE process_emp_audit();
</programlisting>
   </example>

   <para>
    One use of triggers is to maintain a summary table
    of another table. The resulting summary can be used in place of the 
    original table for certain queries &mdash; often with vastly reduced run 
    times.
    This technique is commonly used in Data Warehousing, where the tables
    of measured or observed data (called fact tables) can be extremely large.
    <xref linkend="plpgsql-trigger-summary-example"> shows an example of a
    trigger procedure in <application>PL/pgSQL</application> that maintains
    a summary table for a fact table in a data warehouse.
   </para>


   <example id="plpgsql-trigger-summary-example">
    <title>A <application>PL/pgSQL</application> Trigger Procedure For Maintaining A Summary Table</title>

    <para>
     The schema detailed here is partly based on the <emphasis>Grocery Store
     </emphasis> example from <emphasis>The Data Warehouse Toolkit</emphasis> 
     by Ralph Kimball.
    </para>

<programlisting>
--
-- Main tables - time dimension and sales fact.
--
CREATE TABLE time_dimension (
    time_key                    integer NOT NULL,
    day_of_week                 integer NOT NULL,
    day_of_month                integer NOT NULL,
    month                       integer NOT NULL,
    quarter                     integer NOT NULL,
    year                        integer NOT NULL
);
CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);

CREATE TABLE sales_fact (
    time_key                    integer NOT NULL,
    product_key                 integer NOT NULL,
    store_key                   integer NOT NULL,
    amount_sold                 numeric(12,2) NOT NULL,
    units_sold                  integer NOT NULL,
    amount_cost                 numeric(12,2) NOT NULL
);
CREATE INDEX sales_fact_time ON sales_fact(time_key);

--
-- Summary table - sales by time.
--
CREATE TABLE sales_summary_bytime (
    time_key                    integer NOT NULL,
    amount_sold                 numeric(15,2) NOT NULL,
    units_sold                  numeric(12) NOT NULL,
    amount_cost                 numeric(15,2) NOT NULL
);
CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);

--
-- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
--
CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER AS $maint_sales_summary_bytime$
    DECLARE
        delta_time_key          integer;
        delta_amount_sold       numeric(15,2);
        delta_units_sold        numeric(12);
        delta_amount_cost       numeric(15,2);
    BEGIN

        -- Work out the increment/decrement amount(s).
        IF (TG_OP = 'DELETE') THEN

            delta_time_key = OLD.time_key;
            delta_amount_sold = -1 * OLD.amount_sold;
            delta_units_sold = -1 * OLD.units_sold;
            delta_amount_cost = -1 * OLD.amount_cost;

        ELSIF (TG_OP = 'UPDATE') THEN

            -- forbid updates that change the time_key -
            -- (probably not too onerous, as DELETE + INSERT is how most 
            -- changes will be made).
            IF ( OLD.time_key != NEW.time_key) THEN
                RAISE EXCEPTION 'Update of time_key : % -&gt; % not allowed', OLD.time_key, NEW.time_key;
            END IF;

            delta_time_key = OLD.time_key;
            delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
            delta_units_sold = NEW.units_sold - OLD.units_sold;
            delta_amount_cost = NEW.amount_cost - OLD.amount_cost;

        ELSIF (TG_OP = 'INSERT') THEN

            delta_time_key = NEW.time_key;
            delta_amount_sold = NEW.amount_sold;
            delta_units_sold = NEW.units_sold;
            delta_amount_cost = NEW.amount_cost;

        END IF;


        -- Update the summary row with the new values.
        UPDATE sales_summary_bytime
            SET amount_sold = amount_sold + delta_amount_sold,
                units_sold = units_sold + delta_units_sold,
                amount_cost = amount_cost + delta_amount_cost
            WHERE time_key = delta_time_key;


        -- There might have been no row with this time_key (e.g new data!).
        IF (NOT FOUND) THEN
            BEGIN
                INSERT INTO sales_summary_bytime (
                            time_key, 
                            amount_sold, 
                            units_sold, 
                            amount_cost)
                    VALUES ( 
                            delta_time_key,
                            delta_amount_sold,
                            delta_units_sold,
                            delta_amount_cost
                           );
            EXCEPTION
                --
                -- Catch race condition when two transactions are adding data
                -- for a new time_key.
                --
                WHEN UNIQUE_VIOLATION THEN
                    UPDATE sales_summary_bytime
                        SET amount_sold = amount_sold + delta_amount_sold,
                            units_sold = units_sold + delta_units_sold,
                            amount_cost = amount_cost + delta_amount_cost
                        WHERE time_key = delta_time_key;

            END;
        END IF;
        RETURN NULL;

    END;
$maint_sales_summary_bytime$ LANGUAGE plpgsql;

CREATE TRIGGER maint_sales_summary_bytime
AFTER INSERT OR UPDATE OR DELETE ON sales_fact
    FOR EACH ROW EXECUTE PROCEDURE maint_sales_summary_bytime();
</programlisting>
   </example>

  </sect1>

  <!-- **** Porting from Oracle PL/SQL **** -->

 <sect1 id="plpgsql-porting">
  <title>Porting from <productname>Oracle</productname> PL/SQL</title>

  <indexterm zone="plpgsql-porting">
   <primary>Oracle</primary>
   <secondary>porting from PL/SQL to PL/pgSQL</secondary>
  </indexterm>

  <indexterm zone="plpgsql-porting">
   <primary>PL/SQL (Oracle)</primary>
   <secondary>porting to PL/pgSQL</secondary>
  </indexterm>

  <para>
   This section explains differences between
   <productname>PostgreSQL</>'s <application>PL/pgSQL</application>
   language and Oracle's <application>PL/SQL</application> language,
   to help developers who port applications from
   <trademark class=registered>Oracle</> to <productname>PostgreSQL</>.
  </para>

  <para>
   <application>PL/pgSQL</application> is similar to PL/SQL in many
   aspects. It is a block-structured, imperative language, and all
   variables have to be declared.  Assignments, loops, conditionals
   are similar.  The main differences you should keep in mind when
   porting from <application>PL/SQL</> to
   <application>PL/pgSQL</application> are:

    <itemizedlist>
     <listitem>
      <para>
       There are no default values for parameters in <productname>PostgreSQL</>.
      </para>
     </listitem>

     <listitem>
      <para>
       You can overload function names in <productname>PostgreSQL</>. This is
       often used to work around the lack of default parameters.
      </para>
     </listitem>

     <listitem>
      <para>
       No need for cursors in <application>PL/pgSQL</>, just put the
       query in the <literal>FOR</literal> statement.  (See <xref
       linkend="plpgsql-porting-ex2">.)
      </para>
     </listitem>

     <listitem>
      <para>
       In <productname>PostgreSQL</> you need to use dollar quoting or escape
       single quotes in the function body. See <xref
       linkend="plpgsql-quote-tips">.
      </para>
     </listitem>

     <listitem>
      <para>
       Instead of packages, use schemas to organize your functions
       into groups.
      </para>
     </listitem>

     <listitem>
      <para>
       Since there are no packages, there are no package-level variables
       either. This is somewhat annoying.  You may be able to keep per-session
       state in temporary tables, instead.
      </para>
     </listitem>
    </itemizedlist>
   </para>

  <sect2>
   <title>Porting Examples</title>

   <para>
    <xref linkend="pgsql-porting-ex1"> shows how to port a simple
    function from <application>PL/SQL</> to <application>PL/pgSQL</>.
   </para>

   <example id="pgsql-porting-ex1">
    <title>Porting a Simple Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     Here is an <productname>Oracle</productname> <application>PL/SQL</> function:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name IN varchar,
                                                  v_version IN varchar)
RETURN varchar IS
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
/
show errors;
</programlisting>
    </para>

    <para>
     Let's go through this function and see the differences to <application>PL/pgSQL</>:

     <itemizedlist>
      <listitem>
       <para>
        <productname>Oracle</productname> can have
        <literal>IN</literal>, <literal>OUT</literal>, and
        <literal>INOUT</literal> parameters passed to functions.
        <literal>INOUT</literal>, for example, means that the
        parameter will receive a value and return
        another. <productname>PostgreSQL</> only has <literal>IN</literal>
        parameters, and hence there is no specification of the parameter kind.
       </para>
      </listitem>

      <listitem>
       <para>
        The <literal>RETURN</literal> key word in the function
        prototype (not the function body) becomes
        <literal>RETURNS</literal> in
        <productname>PostgreSQL</productname>.
        Also, <literal>IS</> becomes <literal>AS</>, and you need to
        add a <literal>LANGUAGE</> clause because <application>PL/pgSQL</>
        is not the only possible function language.
       </para>
      </listitem>

      <listitem>
       <para>
        In <productname>PostgreSQL</>, the function body is considered
        to be a string literal, so you need to use quote marks or dollar
        quotes around it.  This substitutes for the terminating <literal>/</>
        in the Oracle approach.
       </para>
      </listitem>

      <listitem>
       <para>
        The <literal>show errors</literal> command does not exist in
        <productname>PostgreSQL</>, and is not needed since errors are
        reported automatically.
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     This is how this function would look when ported to
     <productname>PostgreSQL</>:

<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
                                                  v_version varchar)
RETURNS varchar AS $$
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;
</programlisting>
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex2"> shows how to port a
    function that creates another function and how to handle the
    ensuing quoting problems.
   </para>

   <example id="plpgsql-porting-ex2">
    <title>Porting a Function that Creates Another Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     The following procedure grabs rows from a
     <command>SELECT</command> statement and builds a large function
     with the results in <literal>IF</literal> statements, for the
     sake of efficiency. Notice particularly the differences in the
     cursor and the <literal>FOR</literal> loop.
    </para>

    <para>
     This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
    CURSOR referrer_keys IS 
        SELECT * FROM cs_referrer_keys 
        ORDER BY try_order;

    func_cmd VARCHAR(4000); 
BEGIN 
    func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR,
                 v_domain IN VARCHAR, v_url IN VARCHAR) RETURN VARCHAR IS BEGIN'; 

    FOR referrer_key IN referrer_keys LOOP 
        func_cmd := func_cmd ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ''' || referrer_key.key_string
          || ''' THEN RETURN ''' || referrer_key.referrer_type
          || '''; END IF;'; 
    END LOOP; 

    func_cmd := func_cmd || ' RETURN NULL; END;'; 

    EXECUTE IMMEDIATE func_cmd; 
END; 
/ 
show errors;
</programlisting>
    </para>

    <para>
     Here is how this function would end up in <productname>PostgreSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_update_referrer_type_proc() RETURNS void AS $func$
DECLARE
    referrer_key RECORD;  -- declare a generic record to be used in a FOR
    func_body text;
    func_cmd text;
BEGIN 
    func_body := 'BEGIN' ;

    -- Notice how we scan through the results of a query in a FOR loop
    -- using the FOR &lt;record&gt; construct.

    FOR referrer_key IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
        func_body := func_body ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ' || quote_literal(referrer_key.key_string)
          || ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
          || '; END IF;' ;
    END LOOP; 

    func_body := func_body || ' RETURN NULL; END;';

    func_cmd :=
      'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
                                                        v_domain varchar,
                                                        v_url varchar) 
        RETURNS varchar AS '
      || quote_literal(func_body)
      || ' LANGUAGE plpgsql;' ;

    EXECUTE func_cmd;
    RETURN;
END;
$func$ LANGUAGE plpgsql;
</programlisting>
     Notice how the body of the function is built separately and passed
     through <literal>quote_literal</> to double any quote marks in it.  This
     technique is needed because we cannot safely use dollar quoting for
     defining the new function: we do not know for sure what strings will
     be interpolated from the <structfield>referrer_key.key_string</> field.
     (We are assuming here that <structfield>referrer_key.kind</> can be
     trusted to always be <literal>host</>, <literal>domain</>, or
     <literal>url</>, but <structfield>referrer_key.key_string</> might be
     anything, in particular it might contain dollar signs.) This function
     is actually an improvement on the Oracle original, because it will
     not generate broken code when <structfield>referrer_key.key_string</> or
     <structfield>referrer_key.referrer_type</> contain quote marks.
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex3"> shows how to port a function
    with <literal>OUT</> parameters and string manipulation.
    <productname>PostgreSQL</> does not have an
    <function>instr</function> function, but you can work around it
    using a combination of other
    functions.<indexterm><primary>instr</></indexterm> In <xref
    linkend="plpgsql-porting-appendix"> there is a
    <application>PL/pgSQL</application> implementation of
    <function>instr</function> that you can use to make your porting
    easier.
   </para>

   <example id="plpgsql-porting-ex3">
    <title>Porting a Procedure With String Manipulation and
    <literal>OUT</> Parameters from <application>PL/SQL</> to
    <application>PL/pgSQL</></title>

    <para>
     The following <productname>Oracle</productname> PL/SQL procedure is used
     to parse a URL and return several elements (host, path, and query).
     In <productname>PostgreSQL</>, functions can return only one value.
     One way to work around this is to make the return value a composite
     type (row type).
    </para>

    <para>
     This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_parse_url(
    v_url IN VARCHAR,
    v_host OUT VARCHAR,  -- This will be passed back
    v_path OUT VARCHAR,  -- This one too
    v_query OUT VARCHAR) -- And this one
IS
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;
</programlisting>
    </para>

    <para>
     Here is a possible translation into <application>PL/pgSQL</>:
<programlisting>
CREATE TYPE cs_parse_url_result AS (
    v_host VARCHAR,
    v_path VARCHAR,
    v_query VARCHAR
);

CREATE OR REPLACE FUNCTION cs_parse_url(v_url VARCHAR)
RETURNS cs_parse_url_result AS $$
DECLARE
    res cs_parse_url_result;
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    res.v_host := NULL;
    res.v_path := NULL;
    res.v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN res;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        res.v_host := substr(v_url, a_pos1 + 2);
        res.v_path := '/';
        RETURN res;
    END IF;

    res.v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        res.v_path := substr(v_url, a_pos2);
        RETURN res;
    END IF;

    res.v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    res.v_query := substr(v_url, a_pos1 + 1);
    RETURN res;
END;
$$ LANGUAGE plpgsql;
</programlisting>

     This function could be used like this:
<programlisting>
SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');
</programlisting>
    </para>
   </example>

   <para>
    <xref linkend="plpgsql-porting-ex4"> shows how to port a procedure
    that uses numerous features that are specific to Oracle.
   </para>

   <example id="plpgsql-porting-ex4">
    <title>Porting a Procedure from <application>PL/SQL</> to <application>PL/pgSQL</></title>

    <para>
     The Oracle version:

<programlisting>
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
    a_running_job_count INTEGER;
    PRAGMA AUTONOMOUS_TRANSACTION;<co id="co.plpgsql-porting-pragma">
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;<co id="co.plpgsql-porting-locktable">

    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count &gt; 0 THEN
        COMMIT; -- free lock<co id="co.plpgsql-porting-commit">
        raise_application_error(-20000, 'Unable to create a new job: a job is currently running.');
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, sysdate);
    EXCEPTION
        WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
    END;
    COMMIT;
END;
/
show errors
</programlisting>
   </para>

   <para>
    Procedures like this can easily be converted into <productname>PostgreSQL</>
    functions returning <type>void</type>. This procedure in
    particular is interesting because it can teach us some things:

    <calloutlist>
     <callout arearefs="co.plpgsql-porting-pragma">
      <para>
       There is no <literal>PRAGMA</literal> statement in <productname>PostgreSQL</>.
      </para>
     </callout>

     <callout arearefs="co.plpgsql-porting-locktable">
      <para>
       If you do a <command>LOCK TABLE</command> in <application>PL/pgSQL</>,
       the lock will not be released until the calling transaction is
       finished.
      </para>
     </callout>

     <callout arearefs="co.plpgsql-porting-commit">
      <para>
       You cannot issue <command>COMMIT</> in a
       <application>PL/pgSQL</application> function.  The function is
       running within some outer transaction and so <command>COMMIT</>
       would imply terminating the function's execution.  However, in
       this particular case it is not necessary anyway, because the lock
       obtained by the <command>LOCK TABLE</command> will be released when
       we raise an error.
      </para>
     </callout>
    </calloutlist>
   </para>

   <para>
    This is how we could port this procedure to <application>PL/pgSQL</>:

<programlisting>
CREATE OR REPLACE FUNCTION cs_create_job(v_job_id integer) RETURNS void AS $$
DECLARE
    a_running_job_count integer;
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

    SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

    IF a_running_job_count &gt; 0 THEN
        RAISE EXCEPTION 'Unable to create a new job: a job is currently running';<co id="co.plpgsql-porting-raise">
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
    EXCEPTION
        WHEN unique_violation THEN <co id="co.plpgsql-porting-exception">
            -- don't worry if it already exists
    END;

    RETURN;
END;
$$ LANGUAGE plpgsql;
</programlisting>

    <calloutlist>
     <callout arearefs="co.plpgsql-porting-raise">
      <para>
       The syntax of <literal>RAISE</> is considerably different from
       Oracle's similar statement.
      </para>
     </callout>
     <callout arearefs="co.plpgsql-porting-exception">
      <para>
       The exception names supported by <application>PL/pgSQL</> are
       different from Oracle's.  The set of built-in exception names
       is much larger (see <xref linkend="errcodes-appendix">).  There
       is not currently a way to declare user-defined exception names.
      </para>
     </callout>
    </calloutlist>

    The main functional difference between this procedure and the
    Oracle equivalent is that the exclusive lock on the <literal>cs_jobs</>
    table will be held until the calling transaction completes.  Also, if
    the caller later aborts (for example due to an error), the effects of
    this procedure will be rolled back.
   </para>
   </example>
  </sect2>

  <sect2 id="plpgsql-porting-other">
   <title>Other Things to Watch For</title>

   <para>
    This section explains a few other things to watch for when porting
    Oracle <application>PL/SQL</> functions to
    <productname>PostgreSQL</productname>.
   </para>

   <sect3 id="plpgsql-porting-exceptions">
    <title>Implicit Rollback after Exceptions</title>

    <para>
     In <application>PL/pgSQL</>, when an exception is caught by an
     <literal>EXCEPTION</> clause, all database changes since the block's
     <literal>BEGIN</> are automatically rolled back.  That is, the behavior
     is equivalent to what you'd get in Oracle with

<programlisting>
    BEGIN
        SAVEPOINT s1;
        ... code here ...
    EXCEPTION
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
    END;
</programlisting>

     If you are translating an Oracle procedure that uses
     <command>SAVEPOINT</> and <command>ROLLBACK TO</> in this style,
     your task is easy: just omit the <command>SAVEPOINT</> and
     <command>ROLLBACK TO</>.  If you have a procedure that uses
     <command>SAVEPOINT</> and <command>ROLLBACK TO</> in a different way
     then some actual thought will be required.
    </para>
   </sect3>

   <sect3>
    <title><command>EXECUTE</command></title>

    <para>
     The <application>PL/pgSQL</> version of
     <command>EXECUTE</command> works similarly to the
     <application>PL/SQL</> version, but you have to remember to use
     <function>quote_literal(text)</function> and
     <function>quote_string(text)</function> as described in <xref
     linkend="plpgsql-statements-executing-dyn">.  Constructs of the
     type <literal>EXECUTE 'SELECT * FROM $1';</literal> will not
     work unless you use these functions.
    </para>
   </sect3>

   <sect3 id="plpgsql-porting-optimization">
    <title>Optimizing <application>PL/pgSQL</application> Functions</title>

    <para>
     <productname>PostgreSQL</> gives you two function creation
     modifiers to optimize execution: <quote>volatility</> (whether the
     function always returns the same result when given the same
     arguments) and <quote>strictness</quote> (whether the
     function returns null if any argument is null).  Consult the
     <xref linkend="sql-createfunction"> reference page for details.
    </para>

    <para>
     When making use of these optimization attributes, your
     <command>CREATE FUNCTION</command> statement might look something
     like this:

<programlisting>
CREATE FUNCTION foo(...) RETURNS integer AS $$
...
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
    </para>
   </sect3>
  </sect2>

  <sect2 id="plpgsql-porting-appendix">
   <title>Appendix</title>

   <para>
    This section contains the code for a set of Oracle-compatible
    <function>instr</function> functions that you can use to simplify
    your porting efforts.
   </para>

<programlisting>
--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1, string2, [n], [m]) where [] denotes optional parameters.
-- 
-- Searches string1 beginning at the nth character for the mth occurrence
-- of string2.  If n is negative, search backwards.  If m is not passed,
-- assume 1 (search starts at first character).
--

CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
DECLARE
    pos integer;
BEGIN
    pos:= instr($1, $2, 1);
    RETURN pos;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;


CREATE FUNCTION instr(string varchar, string_to_search varchar, beg_index integer)
RETURNS integer AS $$
DECLARE
    pos integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    length integer;
    ss_length integer;
BEGIN
    IF beg_index &gt; 0 THEN
        temp_str := substring(string FROM beg_index);
        pos := position(string_to_search IN temp_str);

        IF pos = 0 THEN
            RETURN 0;
        ELSE
            RETURN pos + beg_index - 1;
        END IF;
    ELSE
        ss_length := char_length(string_to_search);
        length := char_length(string);
        beg := length + beg_index - ss_length + 2;

        WHILE beg &gt; 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            pos := position(string_to_search IN temp_str);

            IF pos &gt; 0 THEN
                RETURN beg;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;


CREATE FUNCTION instr(string varchar, string_to_search varchar,
                      beg_index integer, occur_index integer)
RETURNS integer AS $$
DECLARE
    pos integer NOT NULL DEFAULT 0;
    occur_number integer NOT NULL DEFAULT 0;
    temp_str varchar;
    beg integer;
    i integer;
    length integer;
    ss_length integer;
BEGIN
    IF beg_index &gt; 0 THEN
        beg := beg_index;
        temp_str := substring(string FROM beg_index);

        FOR i IN 1..occur_index LOOP
            pos := position(string_to_search IN temp_str);

            IF i = 1 THEN
                beg := beg + pos - 1;
            ELSE
                beg := beg + pos;
            END IF;

            temp_str := substring(string FROM beg + 1);
        END LOOP;

        IF pos = 0 THEN
            RETURN 0;
        ELSE
            RETURN beg;
        END IF;
    ELSE
        ss_length := char_length(string_to_search);
        length := char_length(string);
        beg := length + beg_index - ss_length + 2;

        WHILE beg &gt; 0 LOOP
            temp_str := substring(string FROM beg FOR ss_length);
            pos := position(string_to_search IN temp_str);

            IF pos &gt; 0 THEN
                occur_number := occur_number + 1;

                IF occur_number = occur_index THEN
                    RETURN beg;
                END IF;
            END IF;

            beg := beg - 1;
        END LOOP;

        RETURN 0;
    END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
  </sect2>

 </sect1>

</chapter>

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