create_function.7

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.TH "CREATE FUNCTION" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
CREATE FUNCTION \- define a new function

.SH SYNOPSIS
.sp
.nf
CREATE [ OR REPLACE ] FUNCTION
    \fIname\fR ( [ [ \fIargmode\fR ] [ \fIargname\fR ] \fIargtype\fR [, ...] ] )
    [ RETURNS \fIrettype\fR ]
  { LANGUAGE \fIlangname\fR
    | IMMUTABLE | STABLE | VOLATILE
    | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
    | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
    | AS '\fIdefinition\fR'
    | AS '\fIobj_file\fR', '\fIlink_symbol\fR'
  } ...
    [ WITH ( \fIattribute\fR [, ...] ) ]
.sp
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE FUNCTION\fR defines a new function.
\fBCREATE OR REPLACE FUNCTION\fR will either create a
new function, or replace an existing definition.
.PP
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 \fIoverloading\fR).
.PP
To update the definition of an existing function, use
\fBCREATE OR REPLACE FUNCTION\fR. It is not possible
to change the name or argument types of a function this way (if you
tried, you would actually be creating a new, distinct function).
Also, \fBCREATE OR REPLACE FUNCTION\fR will not let
you change the return type of an existing function. To do that,
you must drop and recreate the function. (When using OUT
parameters, that means you can't change the names or types of any
OUT parameters except by dropping the function.)
.PP
If you drop and then recreate a function, the new function is not
the same entity as the old; you will have to drop existing rules, views,
triggers, etc. that refer to the old function. Use
\fBCREATE OR REPLACE FUNCTION\fR to change a function
definition without breaking objects that refer to the function.
.PP
The user that creates the function becomes the owner of the function.
.SH "PARAMETERS"
.TP
\fB\fIname\fB\fR
The name (optionally schema-qualified) of the function to create.
.TP
\fB\fIargmode\fB\fR
The mode of an argument: either IN, OUT,
or INOUT. If omitted, the default is IN.
.TP
\fB\fIargname\fB\fR
The name of an argument. Some languages (currently only PL/pgSQL) let
you use the name in the function body. For other languages the
name of an input argument is just extra documentation. But the name
of an output argument is significant, since it defines the column
name in the result row type. (If you omit the name for an output
argument, the system will choose a default column name.)
.TP
\fB\fIargtype\fB\fR
The data type(s) of the function's arguments (optionally 
schema-qualified), if any. The argument types may be base, composite,
or domain types, or may reference the type of a table column.

Depending on the implementation language it may also be allowed
to specify ``pseudotypes'' such as \fBcstring\fR.
Pseudotypes indicate that the actual argument type is either
incompletely specified, or outside the set of ordinary SQL data types.

The type of a column is referenced by writing
\fItablename\fR.\fIcolumnname\fR%TYPE.
Using this feature can sometimes help make a function independent of
changes to the definition of a table.
.TP
\fB\fIrettype\fB\fR
The return data type (optionally schema-qualified). The return type 
may be a base, composite, or domain type,
or may reference the type of a table column.
Depending on the implementation language it may also be allowed
to specify ``pseudotypes'' such as \fBcstring\fR.
If the function is not supposed to return a value, specify
\fBvoid\fR as the return type.

When there are OUT or INOUT parameters,
the RETURNS clause may be omitted. If present, it
must agree with the result type implied by the output parameters:
RECORD if there are multiple output parameters, or
the same type as the single output parameter.

The SETOF
modifier indicates that the function will return a set of
items, rather than a single item.

The type of a column is referenced by writing
\fItablename\fR.\fIcolumnname\fR%TYPE.
.TP
\fB\fIlangname\fB\fR
The name of the language that the function is implemented in.
May be SQL, C,
internal, or the name of a user-defined
procedural language. For backward compatibility,
the name may be enclosed by single quotes.
.TP
\fBIMMUTABLE\fR
.TP
\fBSTABLE\fR
.TP
\fBVOLATILE\fR
These attributes inform the query optimizer about the behavior
of the function. At most one choice
may be specified. If none of these appear,
VOLATILE is the default assumption.

IMMUTABLE indicates that the function
cannot modify the database and 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.

STABLE indicates that the function
cannot modify the database,
and that within a single table scan it 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 \fBcurrent_timestamp\fR family of functions qualify
as stable, since their values do not change within a transaction.

VOLATILE 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 random(), currval(),
timeofday(). But 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
setval().

For additional details see in the documentation.
.TP
\fBCALLED ON NULL INPUT\fR
.TP
\fBRETURNS NULL ON NULL INPUT\fR
.TP
\fBSTRICT\fR
CALLED ON NULL INPUT (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.

RETURNS NULL ON NULL INPUT or
STRICT 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.
.TP
\fB[EXTERNAL] SECURITY INVOKER\fR
.TP
\fB[EXTERNAL] SECURITY DEFINER\fR
SECURITY INVOKER indicates that the function
is to be executed with the privileges of the user that calls it.
That is the default. SECURITY DEFINER
specifies that the function is to be executed with the
privileges of the user that created it.

The key word EXTERNAL is allowed for SQL
conformance, but it is optional since, unlike in SQL, this feature
applies to all functions not only external ones.
.TP
\fB\fIdefinition\fB\fR
A string constant 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.
.TP
\fB\fIobj_file\fB, \fIlink_symbol\fB\fR
This form of the AS 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 \fIobj_file\fR is the name of the
file containing the dynamically loadable object, and
\fIlink_symbol\fR 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.
.TP
\fB\fIattribute\fB\fR
The historical way to specify optional pieces of information
about the function. The following attributes may appear here:
.RS
.TP
\fBisStrict\fR
Equivalent to STRICT or RETURNS NULL ON NULL INPUT.
.TP
\fBisCachable\fR
isCachable is an obsolete equivalent of
IMMUTABLE; it's still accepted for
backwards-compatibility reasons.
.RE
.PP
Attribute names are not case-sensitive.
.SH "NOTES"
.PP
Refer to in the documentation for further information on writing
functions.
.PP
The full SQL type syntax is allowed for
input arguments and return value. However, some details of the
type specification (e.g., the precision field for
type \fBnumeric\fR) are the responsibility of the
underlying function implementation and are silently swallowed
(i.e., not recognized or
enforced) by the \fBCREATE FUNCTION\fR command.
.PP
PostgreSQL allows function
\fIoverloading\fR; 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).
.PP
Two functions are considered the same if they have the same names and
\fBinput\fR argument types, ignoring any OUT
parameters. Thus for example these declarations conflict:
.sp
.nf
CREATE FUNCTION foo(int) ...
CREATE FUNCTION foo(int, out text) ...
.sp
.fi
.PP
When repeated \fBCREATE FUNCTION\fR calls refer to
the same object file, the file is only loaded once. To unload and
reload the file (perhaps during development), use the LOAD [\fBload\fR(7)] command.
.PP
Use DROP FUNCTION [\fBdrop_function\fR(7)] to remove user-defined
functions.
.PP
It is often helpful to use dollar quoting (see in the documentation) to write the function definition
string, rather than the normal single quote syntax. Without dollar
quoting, any single quotes or backslashes in the function definition must
be escaped by doubling them.
.PP
To be able to define a function, the user must have the
USAGE privilege on the language.
.SH "EXAMPLES"
.PP
Here are some trivial examples to help you get started. For more
information and examples, see in the documentation.
.sp
.nf
CREATE FUNCTION add(integer, integer) RETURNS integer
    AS 'select $1 + $2;'
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;
.sp
.fi
.PP
Increment an integer, making use of an argument name, in
\fBPL/pgSQL\fR:
.sp
.nf
CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$
        BEGIN
                RETURN i + 1;
        END;
$$ LANGUAGE plpgsql;
.sp
.fi
.PP
Return a record containing multiple output parameters:
.sp
.nf
CREATE FUNCTION dup(in int, out f1 int, out f2 text)
    AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
    LANGUAGE SQL;

SELECT * FROM dup(42);
.sp
.fi
You can do the same thing more verbosely with an explicitly named
composite type:
.sp
.nf
CREATE TYPE dup_result AS (f1 int, f2 text);

CREATE FUNCTION dup(int) RETURNS dup_result
    AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
    LANGUAGE SQL;

SELECT * FROM dup(42);
.sp
.fi
.SH "WRITING SECURITY DEFINER FUNCTIONS SAFELY"
.PP
Because a SECURITY DEFINER function is executed
with the privileges of the user that created it, care is needed to
ensure that the function cannot be misused. For security,
search_path should be set to exclude any schemas
writable by untrusted users. This prevents
malicious users from creating objects that mask objects used by the
function. Particularly important in this regard is the
temporary-table schema, which is searched first by default, and
is normally writable by anyone. A secure arrangement can be had
by forcing the temporary schema to be searched last. To do this,
write pg_temp as the last entry in search_path.
This function illustrates safe usage:
.sp
.nf
CREATE FUNCTION check_password(uname TEXT, pass TEXT)
RETURNS BOOLEAN AS $$
DECLARE passed BOOLEAN;
        old_path TEXT;
BEGIN
        -- Save old search_path; notice we must qualify current_setting
        -- to ensure we invoke the right function
        old_path := pg_catalog.current_setting('search_path');

        -- Set a secure search_path: trusted schemas, then 'pg_temp'.
        -- We set is_local = true so that the old value will be restored
        -- in event of an error before we reach the function end.
        PERFORM pg_catalog.set_config('search_path', 'admin, pg_temp', true);

        -- Do whatever secure work we came for.
        SELECT  (pwd = $2) INTO passed
        FROM    pwds
        WHERE   username = $1;

        -- Restore caller's search_path
        PERFORM pg_catalog.set_config('search_path', old_path, true);

        RETURN passed;
END;
$$ LANGUAGE plpgsql SECURITY DEFINER;
.sp
.fi
.SH "COMPATIBILITY"
.PP
A \fBCREATE FUNCTION\fR command is defined in SQL:1999 and later.
The PostgreSQL version is similar but
not fully compatible. The attributes are not portable, neither are the
different available languages.
.PP
For compatibility with some other database systems,
\fIargmode\fR can be written
either before or after \fIargname\fR.
But only the first way is standard-compliant.
.SH "SEE ALSO"
ALTER FUNCTION [\fBalter_function\fR(7)], DROP FUNCTION [\fBdrop_function\fR(l)], GRANT [\fBgrant\fR(l)], LOAD [\fBload\fR(l)], REVOKE [\fBrevoke\fR(l)], createlang [\fBcreatelang\fR(1)]