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MySQL 3.23, 4.0, 4.1 Reference Manual :: 18 Extending MySQL :: 18.2 Adding New Functions to MySQL
18.2.1 Features of the User-Defined Function Interface »
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18.2. Adding New Functions to MySQL

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18.2.1. Features of the User-Defined Function Interface
18.2.2. Adding a New User-Defined Function
18.2.3. Adding a New Native Function

There are two ways to add new functions to MySQL:

  • You can add functions through the user-defined function (UDF) interface. User-defined functions are compiled as object files and then added to and removed from the server dynamically using the CREATE FUNCTION and DROP FUNCTION statements. See Section 12.4.3.1, “CREATE FUNCTION Syntax”.

  • You can add functions as native (built-in) MySQL functions. Native functions are compiled into the mysqld server and become available on a permanent basis.

Each method of creating compiled functions has advantages and disadvantages:

  • If you write user-defined functions, you must install object files in addition to the server itself. If you compile your function into the server, you don't need to do that.

  • Native functions require you to modify a source distribution. UDFs do not. You can add UDFs to a binary MySQL distribution. No access to MySQL source is necessary.

  • If you upgrade your MySQL distribution, you can continue to use your previously installed UDFs, unless you upgrade to a newer version for which the UDF interface changes. (An incompatible change occurred in MySQL 4.1.1 for aggregate functions. A function named xxx_clear() must be defined rather than xxx_reset().) For native functions, you must repeat your modifications each time you upgrade.

Whichever method you use to add new functions, they can be invoked in SQL statements just like native functions such as ABS() or SOUNDEX().

See Section 8.2.3, “Function Name Parsing and Resolution”, for the rules describing how the server interprets references to different kinds of functions.

The following sections describe features of the UDF interface, provide instructions for writing UDFs, discuss security precautions that MySQL takes to prevent UDF misuse, and describe how to add native MySQL functions.

For example source code that illustrates how to write UDFs, take a look at the sql/udf_example.cc file that is provided in MySQL source distributions.