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7.1.1 Definitions

This description uses a narrow definition: The optimizer is the set of routines which decide what execution path the DBMS should take for queries.

MySQL changes these routines frequently, so you should compare what is said here with what's in the current source code. To make that easy, this description includes notes referring to the relevant file and routine, such as See: /sql/select_cc, optimize_cond().

A transformation occurs when one query is changed into another query which delivers the same result. For example, a query could be changed from

SELECT ... WHERE 5 = a 


SELECT ...WHERE a = 5 

Most transformations are less obvious. Some transformations result in faster execution.

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