Way back in 1998, MySQL 3.21 introduced a framework that allowed users to create new SQL functions easily. It made it possible for developers to write their own functions in C/C++ and load them dynamically into the running server. The functions loaded within this framework were called User Defined Functions or UDFs.
Today not much has changed with UDFs, they are more stable and slightly more secure than they used to be, and they can be declared aggregate for use together with
GROUP BY queries. However, many UDFs that worked in 1998 with MySQL 3.21.24 would still work at the time of writing in 2010, with MySQL 5.1.47.
MySQL UDFs are not part of the newer MySQL Plugin API, but there are future plans to make this happen. ...