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 <chapter id="extend">
  <title>Extending <acronym>SQL</acronym></title>

   <indexterm zone="extend">
    <primary>extending SQL</primary>
   </indexterm>

  <para>
   In  the  sections  that follow, we will discuss how you
   can extend the <productname>PostgreSQL</productname> 
   <acronym>SQL</acronym> query language by adding:

   <itemizedlist spacing="compact" mark="bullet">
    <listitem>
     <para>
      functions (starting in <xref linkend="xfunc">)
     </para>
    </listitem>
    <listitem>
     <para>
      aggregates (starting in <xref linkend="xaggr">)
     </para>
    </listitem>
    <listitem>
     <para>
      data types (starting in <xref linkend="xtypes">)
     </para>
    </listitem>
    <listitem>
     <para>
      operators (starting in <xref linkend="xoper">)
     </para>
    </listitem>
    <listitem>
     <para>
      operator classes for indexes (starting in <xref linkend="xindex">)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <sect1 id="extend-how">
   <title>How Extensibility Works</title>

   <para>
    <productname>PostgreSQL</productname> is extensible because its operation  is  
    catalog-driven.   If  you  are familiar with standard 
    relational database systems, you know that  they  store  information
    about  databases,  tables,  columns,  etc., in what are
    commonly known as system catalogs.  (Some systems  call
    this  the data dictionary.)  The catalogs appear to the
    user as tables like any other, but  the  <acronym>DBMS</acronym>  stores
    its  internal  bookkeeping in them.  One key difference
    between <productname>PostgreSQL</productname> and  standard  relational database systems  is
    that <productname>PostgreSQL</productname> stores much more information in its 
    catalogs: not only information about tables and  columns,
    but also information about data types, functions, access
    methods, and so on.  These tables can be  modified  by
    the  user, and since <productname>PostgreSQL</productname> bases its operation 
    on these tables, this means that <productname>PostgreSQL</productname> can  be
    extended   by   users.    By  comparison,  conventional
    database systems can only be extended by changing hardcoded  
    procedures in the source code or by loading modules
    specially written by the <acronym>DBMS</acronym> vendor.
   </para>

   <para>
    The <productname>PostgreSQL</productname> server can moreover
    incorporate user-written code into itself through dynamic loading.
    That is, the user can specify an object code file (e.g., a shared
    library) that implements a new type or function, and
    <productname>PostgreSQL</productname> will load it as required.
    Code written in <acronym>SQL</acronym> is even more trivial to add
    to the server.  This ability to modify its operation <quote>on the
    fly</quote> makes <productname>PostgreSQL</productname> uniquely
    suited for rapid prototyping of new applications and storage
    structures.
   </para>
  </sect1>

  <sect1 id="extend-type-system">
   <title>The <productname>PostgreSQL</productname> Type System</title>

   <indexterm zone="extend-type-system">
    <primary>base type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>base</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>composite type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>composite</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> data types are divided into base
    types, composite types, domains, and pseudo-types.
   </para>

   <sect2>
    <title>Base Types</title>

    <para>
     Base types are those, like <type>int4</type>, that are
     implemented below the level of the <acronym>SQL</> language
     (typically in a low-level language such as C).  They generally
     correspond to what are often known as abstract data types.
     <productname>PostgreSQL</productname> can only operate on such
     types through functions provided by the user and only understands
     the behavior of such types to the extent that the user describes
     them.  Base types are further subdivided into scalar and array
     types.  For each scalar type, a corresponding array type is
     automatically created that can hold variable-size arrays of that
     scalar type.
    </para>
   </sect2>

   <sect2>
    <title>Composite Types</title>

    <para>
     Composite types, or row types, are created whenever the user
     creates a table; it's also possible to define a
     <quote>stand-alone</> composite type with no associated table.  A
     composite type is simply a list of base types with associated
     field names.  A value of a composite type is a row or record of
     field values.  The user can access the component fields from
     <acronym>SQL</> queries.
    </para>
   </sect2>

   <sect2>
    <title>Domains</title>

    <para>
     A domain is based on a particular base type and for many purposes
     is interchangeable with its base type.  However, a domain may
     have constraints that restrict its valid values to a subset of
     what the underlying base type would allow.
    </para>

    <para>
     Domains can be created using the <acronym>SQL</> command
     <xref linkend="sql-createdomain" endterm="sql-createdomain-title">.
     Their creation and use is not discussed in this chapter.
    </para>
   </sect2>

   <sect2>
    <title>Pseudo-Types</title>

    <para>
     There are a few <quote>pseudo-types</> for special purposes.
     Pseudo-types cannot appear as columns of tables or attributes of
     composite types, but they can be used to declare the argument and
     result types of functions.  This provides a mechanism within the
     type system to identify special classes of functions.  <xref
     linkend="datatype-pseudotypes-table"> lists the existing
     pseudo-types.
    </para>
   </sect2>

   <sect2 id="extend-types-polymorphic">
    <title>Polymorphic Types</title>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic type</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic function</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>type</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>function</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

    <para>
     Two pseudo-types of special interest are <type>anyelement</> and
     <type>anyarray</>, which are collectively called <firstterm>polymorphic
     types</>.  Any function declared using these types is said to be
     a <firstterm>polymorphic function</>.  A polymorphic function can
     operate on many different data types, with the specific data type(s)
     being determined by the data types actually passed to it in a particular
     call.
    </para>

    <para>
     Polymorphic arguments and results are tied to each other and are resolved
     to a specific data type when a query calling a polymorphic function is
     parsed.  Each position (either argument or return value) declared as
     <type>anyelement</type> is allowed to have any specific actual
     data type, but in any given call they must all be the
     <emphasis>same</emphasis> actual type. Each 
     position declared as <type>anyarray</type> can have any array data type,
     but similarly they must all be the same type. If there are
     positions declared <type>anyarray</type> and others declared
     <type>anyelement</type>, the actual array type in the
     <type>anyarray</type> positions must be an array whose elements are
     the same type appearing in the <type>anyelement</type> positions.
    </para>

    <para>
     Thus, when more than one argument position is declared with a polymorphic
     type, the net effect is that only certain combinations of actual argument
     types are allowed.  For example, a function declared as
     <literal>equal(anyelement, anyelement)</> will take any two input values,
     so long as they are of the same data type.
    </para>

    <para>
     When the return value of a function is declared as a polymorphic type,
     there must be at least one argument position that is also polymorphic,
     and the actual data type supplied as the argument determines the actual
     result type for that call.  For example, if there were not already
     an array subscripting mechanism, one could define a function that
     implements subscripting as <literal>subscript(anyarray, integer)
     returns anyelement</>.  This declaration constrains the actual first
     argument to be an array type, and allows the parser to infer the correct
     result type from the actual first argument's type.
    </para>
   </sect2>
  </sect1>

  &xfunc;
  &xaggr;
  &xtypes;
  &xoper;
  &xindex;

 </chapter>

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