There are three ways to add a new function to MySQL:
Create a stored function (a type of stored object). A stored function is written using SQL statements rather than by compiling object code. The syntax for writing stored functions is not covered here. See Using Stored Routines.
Create a native (built-in) MySQL function. A native function is added by modifying the MySQL source code to be compiled into the mysqld server and become available on a permanent basis. See Section 6.1, “Adding a Native Function”.
Use the loadable function interface. A loadable function is compiled as a library file and then loaded and unloaded from the server dynamically using the
DROP FUNCTIONstatements. See Section 6.2, “Adding a Loadable Function”.
In some cases, loadable functions are included in component or plugin library files and are loaded and unloaded automatically when the component or plugin is installed or uninstalled.
Loadable functions previously were known as user-defined functions (UDFs). That terminology was something of a misnomer because “user-defined” also can apply to stored functions written using SQL and native functions added by modifying the server source code.
Each method of creating compiled functions has advantages and disadvantages:
Adding a native function requires modifying a source distribution. Adding a loadable function does not; it can be added to a binary MySQL distribution with no access to MySQL source necessary.
A loadable function is contained in an object file that you must install in addition to the server itself. For a function compiled into the server, that is unnecessary. (This point does not apply for loadable functions that are loaded automatically by a component or plugin.)
If you upgrade your MySQL distribution, you can continue to use previously installed loadable functions, unless you upgrade to a newer MySQL version for which the loadable function interface changes. For native functions, you must repeat your source code modifications each time you upgrade.
For the rules describing how the server interprets references to different kinds of functions, see Function Name Parsing and Resolution.
The following sections describe features of the loadable function interface, provide instructions for writing loadable functions, discuss security precautions that MySQL takes to prevent loadable function misuse, and describe how to add native MySQL functions.
For example source code that illustrates how to write loadable
functions, take a look at the
sql/udf_example.cc file that is provided in
MySQL source distributions.
The MySQL source code contains internal documentation written using Doxygen. This documentation is useful for understanding how MySQL works from a developer perspective. The generated Doxygen content is available at https://dev.mysql.com/doc/index-other.html. It is also possible to generate this content locally from a MySQL source distribution using the instructions at Generating MySQL Doxygen Documentation Content.