Question

Currently I am doing mass deletes using variations of the following query:

DELETE FROM t1 WHERE t1.key NOT IN (SELECT t2.key FROM t2)

I am told that sub queries too slow in MySQL and the should be optimized. But I cannot find a better example. Is it possible to do a join and delete?

Answer 1

DELETE FROM t1 USING t1 JOIN t2 ON t1.key = t2.key LIMIT 1000;

And repeat until nothing is left, this allows not blocking table for a long time.

UPD: but if you need to join on the same table this solution will not work.

UPD2: I overlooked the NOT, here is the corrected query:

DELETE FROM t1
USING t1 LEFT JOIN t2 ON t1.key = t2.key
WHERE t2.key IS NULL
LIMIT 1000;

I should note that this is the case where the subquery would perform at the same speed as the JOIN, look into a good post of Quassnoi about the issue.