PostgreSQL 8.2.6 Documentation
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# CREATE FUNCTION

## Name

CREATE FUNCTION -- define a new function

## Synopsis

    CREATE [ OR REPLACE ] FUNCTION
        name ( [ [ argmode ] [ argname ] argtype [, ...] ] )
        [ RETURNS rettype ]
      { LANGUAGE langname
        | IMMUTABLE | STABLE | VOLATILE
        | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
        | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
        | AS 'definition'
        | AS 'obj_file', 'link_symbol'
      } ...
        [ WITH ( attribute [, ...] ) ]

## Description

`CREATE FUNCTION` defines a new function. `CREATE OR REPLACE FUNCTION` will either create a new function, or replace an existing definition.

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 overloading).

To update the definition of an existing function, use `CREATE OR REPLACE FUNCTION`. 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, `CREATE OR REPLACE FUNCTION` 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.)

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 `CREATE OR REPLACE FUNCTION` to change a function definition without breaking objects that refer to the function.

The user that creates the function becomes the owner of the function.

## Parameters

`name`

  The name (optionally schema-qualified) of the function to create.

`argmode`

  The mode of an argument: either `IN`, `OUT`, or `INOUT`. If omitted, the default is `IN`.

`argname`

  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.)

`argtype`

  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 `cstring`. 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 ``tablename`.`columnname`%TYPE`. Using this feature can sometimes help make a function independent of changes to the definition of a table.

`rettype`

  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 `cstring`. If the function is not supposed to return a value, specify `void` 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 ``tablename`.`columnname`%TYPE`.

`langname`

  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.

`IMMUTABLE`
`STABLE`
`VOLATILE`

  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 `current_timestamp` 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 Section 33.6.

`CALLED ON NULL INPUT`
`RETURNS NULL ON NULL INPUT`
`STRICT`

  `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.

`[EXTERNAL] SECURITY INVOKER`
`[EXTERNAL] SECURITY DEFINER`

  `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.

`definition`

  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.

``obj_file`, `link_symbol``

  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 `obj_file` is the name of the file containing the dynamically loadable object, and `link_symbol` 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.

`attribute`

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

  `isStrict`

    Equivalent to `STRICT` or `RETURNS NULL ON NULL INPUT`.

  `isCachable`

    `isCachable` is an obsolete equivalent of `IMMUTABLE`; it's still accepted for backwards-compatibility reasons.

  Attribute names are not case-sensitive.

## Notes

Refer to Section 33.3 for further information on writing functions.

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 `numeric`) are the responsibility of the underlying function implementation and are silently swallowed (i.e., not recognized or enforced) by the `CREATE FUNCTION` command.

PostgreSQL allows function overloading; 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).

Two functions are considered the same if they have the same names and input argument types, ignoring any `OUT` parameters. Thus for example these declarations conflict:

    CREATE FUNCTION foo(int) ...
    CREATE FUNCTION foo(int, out text) ...

When repeated `CREATE FUNCTION` 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 command.

Use DROP FUNCTION to remove user-defined functions.

It is often helpful to use dollar quoting (see Section 4.1.2.2) 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.

To be able to define a function, the user must have the `USAGE` privilege on the language.

## Examples

Here are some trivial examples to help you get started. For more information and examples, see Section 33.3.

    CREATE FUNCTION add(integer, integer) RETURNS integer
        AS 'select $1 + $2;'
        LANGUAGE SQL
        IMMUTABLE
        RETURNS NULL ON NULL INPUT;

Increment an integer, making use of an argument name, in PL/pgSQL:

    CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$
            BEGIN
                    RETURN i + 1;
            END;
    $$ LANGUAGE plpgsql;

Return a record containing multiple output parameters:

    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);

You can do the same thing more verbosely with an explicitly named composite type:

    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);

## Writing `SECURITY DEFINER` Functions Safely

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:

    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;

## Compatibility

A `CREATE FUNCTION` 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.

For compatibility with some other database systems, `argmode` can be written either before or after `argname`. But only the first way is standard-compliant.

## See Also

ALTER FUNCTION, DROP FUNCTION, GRANT, LOAD, REVOKE, createlang

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