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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. If a schema name is included,
then the function is created in the
specified schema. Otherwise it is created in the current schema (the
one at the front of the search path; see <literal>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>
</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 (including the schema name) 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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