At the MySQL User's conference 2008 I did a tutorial on MySQL User-defined functions. At the same conference, Brian Miezejewski and Mark Atwood did a talk on aggregate UDFs.

In relation their upcoming talk, Mark wrote a blog post reporting that MySQL Aggregate UDFs are three times slower than equivalent native functions:I just proved to myself via testing and benchmarking that user defined aggregate functions are about three times slower than the call interface to the built-in functions.Later on he posted an update, …

Yesterday, I was on the freenode ##pentaho irc channel when Andres Chaves asked me how to calculate the Nth percentile in MySQL. He saw a solution somewhere using subqueries, but wasn't too happy about it.

A while ago I wrote about calulating the median in MySQL, and it turns out the Nth percentile can be calculated using a similar, single-pass approach, not relying on subqueries, UDFs, or user-defined variables.
The percentile....
So, what is a percentile exactly? Here's what the wikipedia says:

A percentile is the value of a variable below which a certain percent of observations fall. So the 20th percentile is the value (or score) below which 20 …

The MySQL User's conference will be held in less than a month from now!!!

This year there is quite a good number of sessions on adding your own functions and procedures, such as:

• 
  
• Advanced Stored Procedures
  
• A Tour of External Language Stored Procedures for MySQL
  
• Code Generators for MySQL Plugins and User Defined Functions (UDFs)
  
• …