This section describes the optional precondition queries. Precondition queries determine that specific conditions are true before executing the main query. For example, they are used in the default advisors to check the MySQL server version, because some main queries cannot be executed on older versions of the server. The following is an example of a precondition query which checks the version of the MySQL server:
<precondition> <![CDATA[SELECT @@version NOT LIKE '5.0%' AND @@version NOT LIKE '5.1%']]> </precondition>
If the server version is higher than 5.1, the precondition returns true and the main query is executed. If the MySQL server is version 5.0.x or 5.1.x, the precondition returns false and the main query is not executed.