Question

This query takes 10 seconds to complete. But when I manually perform the subquery and change the t1.id restriction to that list, it’s done in 0.00 seconds. What can I do to let MySQL execute the query quicker?

SELECT t1.col1, t2.col2, t3.col3
FROM t1, t2, t3
WHERE  t1.t2id = t2.id AND t1.t3id = t3.id
AND t1.id IN ( SELECT id FROM t4 WHERE blah = 123 )

Also, why is this happening? I suppose MySQL joins all three tables in some way before filtering on t1.id.

t1, t2 and t3 contain 3000, 15 and 80 rows, respectively. The subquery returns 2-10 rows.

Answer 1

Rewrite the query without subquery:

SELECT t1.col1, t2.col2, t3.col3
FROM t1, t2, t3, (SELECT id FROM t4 WHERE blah = 123) AS t4
WHERE  t1.t2id = t2.id AND t1.t3id = t3.id
AND t1.id=t4.id

Be sure you have indexes on the fields used in the WHERE clauses.