13.7.4.1 CREATE FUNCTION Syntax for User-Defined Functions

A user-defined function (UDF) is a way to extend MySQL with a new function that works like a native (built-in) MySQL function such as ABS() or CONCAT().

function_name is the name that should be used in SQL statements to invoke the function. The RETURNS clause indicates the type of the function's return value. DECIMAL is a legal value after RETURNS, but currently DECIMAL functions return string values and should be written like STRING functions.

shared_library_name is the base name of the shared library file that contains the code that implements the function. The file must be located in the plugin directory. This directory is given by the value of the plugin_dir system variable. For more information, see Section 5.7.1, “Installing and Uninstalling User-Defined Functions”.

To create a function, you must have the INSERT privilege for the mysql system database. This is necessary because CREATE FUNCTION adds a row to the mysql.func system table that records the function's name, type, and shared library name.

UDFs registered using CREATE FUNCTION are listed in the Performance Schema user_defined_functions table; see Section 26.12.18.6, “The user_defined_functions Table”.

An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION. All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable.

For instructions on writing user-defined functions, see Section 29.4.2, “Adding a New User-Defined Function”. For the UDF mechanism to work, functions must be written in C or C++ (or another language that can use C calling conventions), your operating system must support dynamic loading and you must have compiled mysqld dynamically (not statically).

An AGGREGATE function works exactly like a native MySQL aggregate (summary) function such as SUM or COUNT().