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Every select is performed in these base steps:
JOIN::prepare
Initialization and linking JOIN structure to st_select_lex.
JOIN
st_select_lex
fix_fields() for all items (after fix_fields(), we know everything about item).
fix_fields()
Moving HAVING to WHERE if possible.
HAVING
WHERE
Initialization procedure if there is one.
JOIN::optimize
Single select optimization.
Creation of first temporary table if needed.
JOIN::exec
Performing select (a second temporary table may be created).
JOIN::cleanup
Removing all temporary tables, other cleanup.
JOIN::reinit
Prepare all structures for execution of SELECT (with JOIN::exec).
SELECT
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