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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_function.sgml,v 1.36 2002/04/11 19:59:55 tgl Exp $
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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>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>

     <varlistentry>
      <term>isImmutable</term>
      <term>isCachable</term>
      <term>isStable</term>
      <term>isVolatile</term>
      <listitem>
       <para>
        These attributes inform the system whether it is safe to replace
	multiple evaluations of the function with a single evaluation.
	At most one choice should be specified.  (If none of these appear,
	<option>isVolatile</option> is the default assumption.)
	<option>isImmutable</option> 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 parameter list.  If this option is given,
	any call of the function with all-constant arguments can be
	immediately replaced with the function value.
	<option>isCachable</option> is an
	obsolete equivalent of <option>isImmutable</option>; it's still
	accepted for backwards-compatibility reasons.
	<option>isStable</option> 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 timezone), etc.  Also note
	that the <literal>CURRENT_TIMESTAMP</> family of functions qualify
	as stable, since their values do not change within a transaction.
	<option>isVolatile</option> 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>implicitCoercion</term>
      <listitem>
       <para>
	<option>implicitCoercion</option> indicates that the function 
	may be used for implicit type conversions.
	See <xref linkend="coercion-functions"
	endterm="coercion-functions-title"> for more detail.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    Attribute names are not case-sensitive.
  </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>PostgreSQL</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>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.  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>

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

  <para>
   By default, only the owner (creator) of the function has the right
   to execute it.  Other users must be granted the
   <literal>EXECUTE</literal> privilege on the function to be able to
   use it.
  </para>
 </refsect1>

 <refsect1 id="COERCION-FUNCTIONS">
  <refsect1info>
   <date>2002-04-11</date>
  </refsect1info>
  <title id="coercion-functions-title">
   Type Coercion Functions
  </title>
  <para>
   A function that has one parameter and is named the same as its output
   datatype is considered to be a <firstterm>type coercion function</>:
   it can be invoked to convert a value of its input datatype into a value
   of its output datatype.  For example,
<programlisting>
	SELECT CAST(42 AS text);
</programlisting>
   converts the integer constant 42 to text by invoking a function
   <literal>text(int4)</>, if such a function exists and returns type
   text.  (If no suitable conversion function can be found, the cast fails.)
  </para>

  <para>
   If a potential coercion function is marked <literal>implicitCoercion</>,
   then it can be invoked in any context where the conversion it defines
   is required.  Coercion functions not so marked can be invoked only by
   explicit <literal>CAST</>,
   <replaceable>x</><literal>::</><replaceable>typename</>,
   or <replaceable>typename</>(<replaceable>x</>) constructs.
   For example, supposing that foo.f1 is a column of type text, then
<programlisting>
	INSERT INTO foo(f1) VALUES(42);
</programlisting>
   will be allowed if <literal>text(int4)</> is marked
   <literal>implicitCoercion</>, otherwise not.
  </para>

  <para>
   It is wise to be conservative about marking coercion functions as
   implicit coercions.  An overabundance of implicit coercion paths
   can cause <productname>PostgreSQL</productname> to choose surprising
   interpretations of commands, 
   or to be unable to resolve commands at all because there are multiple
   possible interpretations.  A good rule of thumb is to make coercions
   implicitly invokable only for information-preserving transformations
   between types in the same general type category.  For example, int2
   to int4 coercion can reasonably be implicit, but be wary of marking
   int4 to text or float8 to int4 as implicit coercions.
  </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 from the
   user-defined type complex to the built-in 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 absolute file name 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 WITH (isStrict);
</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>

   Note that the function is marked <quote>strict</>; this allows us
   to skip checking for NULL input in the function body.
  </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-grant">,
   <xref linkend="sql-load">,
   <xref linkend="sql-revoke">,
   <citetitle>PostgreSQL Programmer's Guide</citetitle>
  </para>
 </refsect1>

</refentry>

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