24.3. Adding New Procedures to MySQL

In MySQL, you can define a procedure in C++ that can access and modify the data in a query before it is sent to the client. The modification can be done on a row-by-row or GROUP BY level.

We have created an example procedure to show you what can be done.

Additionally, we recommend that you take a look at mylua. With this you can use the LUA language to load a procedure at runtime into mysqld.

24.3.1. Procedure Analyse

analyse([max_elements[,max_memory]])

This procedure is defined in the sql/sql_analyse.cc file. It examines the result from a query and returns an analysis of the results that suggests optimal data types for each column. To obtain this analysis, append PROCEDURE ANALYSE to the end of a SELECT statement:

SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max_elements,[max_memory]])

For example:

SELECT col1, col2 FROM table1 PROCEDURE ANALYSE(10, 2000);

The results show some statistics for the values returned by the query, and propose an optimal data type for the columns. This can be helpful for checking your existing tables, or after importing new data. You may need to try different settings for the arguments so that PROCEDURE ANALYSE() does not suggest the ENUM data type when it is not appropriate.

The arguments are optional and are used as follows:

max_elements (default 256) is the maximum number of distinct values that analyse notices per column. This is used by analyse to check whether the optimal data type should be of type ENUM.
max_memory (default 8192) is the maximum amount of memory that analyse should allocate per column while trying to find all distinct values.

24.3.2. Writing a Procedure

For the moment, the only documentation for this is the source.

You can find all information about procedures by examining the following files:

sql/sql_analyse.cc
sql/procedure.h
sql/procedure.cc
sql/sql_select.cc