create_function.sgml

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<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>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">argtype</replaceable> [, ...] ] )
    RETURNS <replaceable class="parameter">rettype</replaceable>
    AS '<replaceable class="parameter">definition</replaceable>'
    LANGUAGE <replaceable class="parameter">langname</replaceable>
    [ WITH ( <replaceable class="parameter">attribute</replaceable> [, ...] ) ]
CREATE [ OR REPLACE ] FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">argtype</replaceable> [, ...] ] )
    RETURNS <replaceable class="parameter">rettype</replaceable>
    AS '<replaceable class="parameter">obj_file</replaceable>', '<replaceable class="parameter">link_symbol</replaceable>'
    LANGUAGE <replaceable class="parameter">langname</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.

   <variablelist>
    <title>Parameters</title>

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

     <listitem>
      <para>
       The name of a function to create.  The name need not be unique,
       because functions may be overloaded, but functions with the
       same name must have different argument types.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       The data type(s) of the function's arguments, if any.  The
       input types may be base or complex types,
       <literal>opaque</literal>, or the same as the type of an
       existing column.  <literal>Opaque</literal> indicates
       that the function accepts arguments of a non-SQL type such as
       <type>char *</type>.
	The type of a column is indicated using <replaceable
	class="parameter">tablename</replaceable>.<replaceable
	class="parameter">columnname</replaceable><literal>%TYPE</literal>;
	using this can sometimes help make a function independent from
	changes to the definition of a table.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       The return data type.  The output type may be specified as a
       base type, complex type, <literal>setof</literal> type,
       <literal>opaque</literal>, or the same as the type of an
       existing column.
       The <literal>setof</literal>
       modifier indicates that the function will return a set of
       items, rather than a single item.  Functions with a declared
       return type of <literal>opaque</literal> do not return a value.
       These cannot be called directly; trigger functions make use of
       this feature.
      </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 query, or text in a procedural language.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       This form of the <literal>AS</literal> clause is used for
       dynamically linked 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
       object's link symbol, that is, the name of the function in the C
       language source code.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       May be <literal>SQL</literal>, <literal>C</literal>,
       <literal>internal</literal>, or <replaceable
       class="parameter">plname</replaceable>, where <replaceable
       class="parameter">plname</replaceable> is the name of a
       created procedural language. See
       <xref linkend="sql-createlanguage" endterm="sql-createlanguage-title">
       for details.  For backward compatibility, the name may be
       enclosed by single quotes.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       An optional piece of information about the function, used for
       optimization.  See below for details.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

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

  <para>
    The following attributes may appear in the WITH clause:

    <variablelist>
     <varlistentry>
      <term>iscachable</term>
      <listitem>
       <para>
	<option>Iscachable</option> indicates that the function always
	returns the same result when given the same argument values (i.e.,
	it does not do database lookups or otherwise use information not
	directly present in its parameter list).  The optimizer uses
	<option>iscachable</option> to know whether it is safe to
	pre-evaluate a call of the function.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>isstrict</term>
      <listitem>
       <para>
	<option>isstrict</option> indicates that the function always
	returns NULL whenever any of its arguments are NULL.  If this
	attribute is specified, the function is not executed when there
	are NULL arguments; instead a NULL result is assumed automatically.
	When <option>isstrict</option> is not specified, the function will
	be called for NULL inputs.  It is then the function author's
	responsibility to check for NULLs if necessary and respond
	appropriately.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
  </para>

 </refsect1>

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

   <para>
    Refer to the chapter in the
    <citetitle>PostgreSQL Programmer's Guide</citetitle>
    on the topic of extending
    <productname>Postgres</productname> via functions 
    for further information on writing external 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>numeric</type> types) 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>Postgres</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.  This facility must
    be used with caution for internal and C-language functions, however.    
   </para>

   <para>
    Two <literal>internal</literal>
    functions cannot have the same C name without causing
    errors at link time.  To get around that, give them different C names
    (for example, use the argument types as part of the C names), then
    specify those names in the AS clause of <command>CREATE FUNCTION</command>.
    If the AS clause is left empty, then <command>CREATE FUNCTION</command>
    assumes the C name of the function is the same as the SQL name.
   </para>

   <para>
    Similarly, when overloading SQL function names with multiple C-language
    functions, give
    each C-language instance of the function a distinct name, then use
    the alternative form of the <command>AS</command> clause in the
    <command>CREATE FUNCTION</command> syntax to select the appropriate
    C-language implementation of each overloaded SQL function.
   </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>
    To update the definition of an existing function, use
    <command>CREATE OR REPLACE FUNCTION</command>.  Note that 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 re-create the function.
   </para>

   <para>
    If you drop and then re-create 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>

 </refsect1>

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

  <para>
   To create a simple SQL function:

<programlisting>
CREATE FUNCTION one() RETURNS integer
    AS 'SELECT 1 AS RESULT;'
    LANGUAGE SQL;

SELECT one() AS answer;
<computeroutput>
 answer 
--------
      1
</computeroutput>
</programlisting>
  </para>

  <para>
   The next example creates a C function by calling a routine from a
   user-created shared library named <filename>funcs.so</> (the extension
   may vary across platforms).  The shared library file is sought in the
   server's dynamic library search path.  This particular routine calculates
   a check digit and returns TRUE if the check digit in the function
   parameters is correct.  It is intended for use in a CHECK
   constraint.

<programlisting>
CREATE FUNCTION ean_checkdigit(char, char) RETURNS boolean
    AS 'funcs' LANGUAGE C;
    
CREATE TABLE product (
    id        char(8) PRIMARY KEY,
    eanprefix char(8) CHECK (eanprefix ~ '[0-9]{2}-[0-9]{5}')
                      REFERENCES brandname(ean_prefix),
    eancode   char(6) CHECK (eancode ~ '[0-9]{6}'),
    CONSTRAINT ean    CHECK (ean_checkdigit(eanprefix, eancode))
);
</programlisting>
  </para>

  <para>
   This example creates a function that does type conversion between the
   user-defined type complex, and the internal type point.  The
   function is implemented by a dynamically loaded object that was
   compiled from C source (we illustrate the now-deprecated alternative
   of specifying the exact pathname to the shared object file).
   For <productname>PostgreSQL</productname> to
   find a type conversion function automatically, the SQL function has
   to have the same name as the return type, and so overloading is
   unavoidable.  The function name is overloaded by using the second
   form of the <command>AS</command> clause in the SQL definition:

<programlisting>
CREATE FUNCTION point(complex) RETURNS point
    AS '/home/bernie/pgsql/lib/complex.so', 'complex_to_point'
    LANGUAGE C;
</programlisting>

  The C declaration of the function could be:

<programlisting>
Point * complex_to_point (Complex *z)
{
	Point *p;

	p = (Point *) palloc(sizeof(Point));
	p->x = z->x;
	p->y = z->y;
		
	return p;
}
</programlisting>
  </para>    
 </refsect1>

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

  <para>
   A <command>CREATE FUNCTION</command> command is defined in SQL99.
   The <application>PostgreSQL</application> version is similar but
   not 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-dropfunction">,
   <xref linkend="sql-load">,
   <citetitle>PostgreSQL Programmer's Guide</citetitle>
  </para>
 </refsect1>

</refentry>

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