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

   <indexterm zone="type-system">
    <primary>extending SQL</primary>
    <secondary>types</secondary>
   </indexterm>

   <indexterm zone="type-system">
    <primary>data types</primary>
   </indexterm>

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

   <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>

   <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>

   <para>
    A domain type 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.  Domains can
    be created by simple <acronym>SQL</> commands.
   </para>

   <para>
    Finally, there are a few <quote>pseudo-types</> for special purposes.
    Pseudo-types cannot appear as fields of tables or 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 id="types-polymorphic">
    <title>Polymorphic Types and Functions</title>

   <indexterm>
    <primary>polymorphic types</primary>
   </indexterm>

   <indexterm>
    <primary>polymorphic functions</primary>
   </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>foo(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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