<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.55 2003/11/29 19:51:38 pgsql Exp $
-->

<refentry id="SQL-CREATEFUNCTION">
 <refmeta>
  <refentrytitle id="SQL-CREATEFUNCTION-TITLE">CREATE FUNCTION</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE FUNCTION</refname>
  <refpurpose>define a new function</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createfunction">
  <primary>CREATE FUNCTION</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">argtype</replaceable> [, ...] ] )
    RETURNS <replaceable class="parameter">rettype</replaceable>
  { LANGUAGE <replaceable class="parameter">langname</replaceable>
    | IMMUTABLE | STABLE | VOLATILE
    | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
    | [EXTERNAL] SECURITY INVOKER | [EXTERNAL] SECURITY DEFINER
    | AS '<replaceable class="parameter">definition</replaceable>'
    | AS '<replaceable class="parameter">obj_file</replaceable>', '<replaceable class="parameter">link_symbol</replaceable>'
  } ...
    [ WITH ( <replaceable class="parameter">attribute</replaceable> [, ...] ) ]
</synopsis>
 </refsynopsisdiv>
  
 <refsect1 id="sql-createfunction-description">
  <title>Description</title>

  <para>
   <command>CREATE FUNCTION</command> defines a new function.
   <command>CREATE OR REPLACE FUNCTION</command> will either create a
   new function, or replace an existing definition.
  </para>

  <para>
   If a schema name is included, then the function is created in the
   specified schema.  Otherwise it is created in the current schema.
   The name of the new function must not match any existing function
   with the same argument types in the same schema.  However,
   functions of different argument types may share a name (this is
   called <firstterm>overloading</>).
  </para>

  <para>
   To update the definition of an existing function, use
   <command>CREATE OR REPLACE FUNCTION</command>.  It is not possible
   to change the name or argument types of a function this way (if you
   tried, you'd just be creating a new, distinct function).  Also,
   <command>CREATE OR REPLACE FUNCTION</command> will not let you
   change the return type of an existing function.  To do that, you
   must drop and recreate the function.
  </para>

  <para>
   If you drop and then recreate a function, the new function is not
   the same entity as the old; you will break existing rules, views,
   triggers, etc. that referred to the old function.  Use
   <command>CREATE OR REPLACE FUNCTION</command> to change a function
   definition without breaking objects that refer to the function.
  </para>

  <para>
   The user that creates the function becomes the owner of the function.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

   <variablelist>

    <varlistentry>
     <term><replaceable class="parameter">name</replaceable></term>

     <listitem>
      <para>
       The name of a function to create.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">argtype</replaceable></term>

     <listitem>
      <para>
       The data type(s) of the function's arguments (optionally 
       schema-qualified), if any. The argument types may be base, complex, or 
       domains, or copy the type of an existing column.
      </para>
      <para>
       The type of a column is referenced by writing
       <literal><replaceable
       class="parameter">tablename</replaceable>.<replaceable
       class="parameter">columnname</replaceable>%TYPE</literal>;
       using this can sometimes help make a function independent from
       changes to the definition of a table.
      </para>
      <para>
       Depending on the implementation language it may also be allowed
       to specify <quote>pseudotypes</> such as <type>cstring</>.
       Pseudotypes indicate that the actual argument type is either
       incompletely specified, or outside the set of ordinary SQL data types.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">rettype</replaceable></term>

     <listitem>
      <para>
       The return data type (optionally schema-qualified). The return type 
       may be a base type, complex type, or a domain,
       or may be specified to copy the type of an existing column. See the description
       under <literal>argtype</literal> above on how to reference the type
       of an existing column.
      </para>
      <para>
       Depending on the implementation language it may also be allowed
       to specify <quote>pseudotypes</> such as <type>cstring</>.
       The <literal>SETOF</literal>
       modifier indicates that the function will return a set of
       items, rather than a single item.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">langname</replaceable></term>

     <listitem>
      <para>
       The name of the language that the function is implemented in.
       May be <literal>SQL</literal>, <literal>C</literal>,
       <literal>internal</literal>, or the name of a user-defined
       procedural language.  (See also <xref linkend="app-createlang"
       endterm="app-createlang-title">.)  For backward compatibility,
       the name may be enclosed by single quotes.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>IMMUTABLE</literal></term>
     <term><literal>STABLE</literal></term>
     <term><literal>VOLATILE</literal></term>

     <listitem>
      <para>
       These attributes inform the system whether it is safe to
       replace multiple evaluations of the function with a single
       evaluation, for run-time optimization.  At most one choice
       should be specified.  If none of these appear,
       <literal>VOLATILE</literal> is the default assumption.
      </para>

      <para>
       <literal>IMMUTABLE</literal> indicates that the function always
       returns the same result when given the same argument values; that
       is, it does not do database lookups or otherwise use information not
       directly present in its argument list.  If this option is given,
       any call of the function with all-constant arguments can be
       immediately replaced with the function value.
      </para>

      <para>
       <literal>STABLE</literal> indicates that within a single table scan
       the function will consistently
       return the same result for the same argument values, but that its
       result could change across SQL statements.  This is the appropriate
       selection for functions whose results depend on database lookups,
       parameter variables (such as the current time zone), etc.  Also note
       that the <function>current_timestamp</> family of functions qualify
       as stable, since their values do not change within a transaction.
      </para>

      <para>
       <literal>VOLATILE</literal> indicates that the function value can
       change even within a single table scan, so no optimizations can be
       made.  Relatively few database functions are volatile in this sense;
       some examples are <literal>random()</>, <literal>currval()</>,
       <literal>timeofday()</>.  Note that any function that has side-effects
       must be classified volatile, even if its result is quite predictable,
       to prevent calls from being optimized away; an example is
       <literal>setval()</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>CALLED ON NULL INPUT</literal></term>
     <term><literal>RETURNS NULL ON NULL INPUT</literal></term>
     <term><literal>STRICT</literal></term>

     <listitem>
      <para>
       <literal>CALLED ON NULL INPUT</literal> (the default) indicates
       that the function will be called normally when some of its
       arguments are null.  It is then the function author's
       responsibility to check for null values if necessary and respond
       appropriately.
      </para>

      <para>
       <literal>RETURNS NULL ON NULL INPUT</literal> or
       <literal>STRICT</literal> indicates that the function always
       returns null whenever any of its arguments are null.  If this
       parameter is specified, the function is not executed when there
       are null arguments; instead a null result is assumed
       automatically.
      </para>
     </listitem>
    </varlistentry>

   <varlistentry>
    <term><literal><optional>EXTERNAL</optional> SECURITY INVOKER</literal></term>
    <term><literal><optional>EXTERNAL</optional> SECURITY DEFINER</literal></term>

    <listitem>
     <para>
      <literal>SECURITY INVOKER</literal> indicates that the function
      is to be executed with the privileges of the user that calls it.
      That is the default.  <literal>SECURITY DEFINER</literal>
      specifies that the function is to be executed with the
      privileges of the user that created it.
     </para>

     <para>
      The key word <literal>EXTERNAL</literal> is present for SQL
      conformance but is optional since, unlike in SQL, this feature
      does not only apply to external functions.
     </para>
    </listitem>
   </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">definition</replaceable></term>

     <listitem>
      <para>
       A string defining the function; the meaning depends on the
       language.  It may be an internal function name, the path to an
       object file, an SQL command, or text in a procedural language.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal><replaceable class="parameter">obj_file</replaceable>, <replaceable class="parameter">link_symbol</replaceable></literal></term>

     <listitem>
      <para>
       This form of the <literal>AS</literal> clause is used for
       dynamically loadable C language functions when the function name
       in the C language source code is not the same as the name of
       the SQL function. The string <replaceable
       class="parameter">obj_file</replaceable> is the name of the
       file containing the dynamically loadable object, and
       <replaceable class="parameter">link_symbol</replaceable> is the
       function's link symbol, that is, the name of the function in the C
       language source code.  If the link symbol is omitted, it is assumed
       to be the same as the name of the SQL function being defined.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">attribute</replaceable></term>

     <listitem>
      <para>
       The historical way to specify optional pieces of information
       about the function.  The following attributes may appear here:

      <variablelist>
       <varlistentry>
        <term><literal>isStrict</></term>
        <listitem>
         <para>
          Equivalent to <literal>STRICT</literal> or <literal>RETURNS NULL ON NULL INPUT</literal>
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>isCachable</></term>
        <listitem>
         <para>
          <literal>isCachable</literal> is an obsolete equivalent of
          <literal>IMMUTABLE</literal>; it's still accepted for
          backwards-compatibility reasons.
         </para>
        </listitem>
       </varlistentry>

      </variablelist>

      Attribute names are not case-sensitive.
     </para>
    </listitem>
   </varlistentry>

   </variablelist>

 </refsect1>

 <refsect1 id="sql-createfunction-notes">
  <title>Notes</title>

   <para>
    Refer to <xref linkend="xfunc"> for further information on writing
    functions.
   </para>

   <para>
    The full <acronym>SQL</acronym> type syntax is allowed for
    input arguments and return value. However, some details of the
    type specification (e.g., the precision field for
    type <type>numeric</type>) are the responsibility of the
    underlying function implementation and are silently swallowed
    (i.e., not recognized or
    enforced) by the <command>CREATE FUNCTION</command> command.
   </para>

   <para>
    <productname>PostgreSQL</productname> allows function
    <firstterm>overloading</firstterm>; that is, the same name can be
    used for several different functions so long as they have distinct
    argument types.  However, the C names of all functions must be
    different, so you must give overloaded C functions different C
    names (for example, use the argument types as part of the C
    names).
   </para>

   <para>
    When repeated <command>CREATE FUNCTION</command> calls refer to
    the same object file, the file is only loaded once.  To unload and
    reload the file (perhaps during development), use the <xref
    linkend="sql-load" endterm="sql-load-title"> command.
   </para>

   <para>
    Use <command>DROP FUNCTION</command>
    to remove user-defined functions.
   </para>

  <para>
   Any single quotes or backslashes in the function definition must be
   escaped by doubling them.
  </para>

   <para>
    To be able to define a function, the user must have the
    <literal>USAGE</literal> privilege on the language.
   </para>

 </refsect1>

 <refsect1 id="sql-createfunction-examples">
  <title>Examples</title>

  <para>
   Here is a trivial example to help you get started.  For more
   information and examples, see <xref linkend="xfunc">.
<programlisting>
CREATE FUNCTION add(integer, integer) RETURNS integer
    AS 'select $1 + $2;'
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;
</programlisting>
  </para>
 </refsect1>

 
 <refsect1 id="sql-createfunction-compat">
  <title>Compatibility</title>

  <para>
   A <command>CREATE FUNCTION</command> command is defined in SQL99.
   The <productname>PostgreSQL</productname> version is similar but
   not fully compatible.  The attributes are not portable, neither are the
   different available languages.
  </para>
 </refsect1>


 <refsect1 id="sql-createfunction-seealso">
  <title>See Also</title>

  <para>
   <xref linkend="sql-alterfunction" endterm="sql-alterfunction-title">,
   <xref linkend="sql-dropfunction" endterm="sql-dropfunction-title">,
   <xref linkend="sql-grant" endterm="sql-grant-title">,
   <xref linkend="sql-load" endterm="sql-load-title">,
   <xref linkend="sql-revoke" endterm="sql-revoke-title">,
   <xref linkend="app-createlang">
  </para>
 </refsect1>

</refentry>

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