Question

Can I somehow force mysql to do a full scan on the subquery result instead of the existing table?

Let’s say I’ve got a query:

DELETE FROM table_a WHERE id IN (SELECT id FROM table_b WHERE ...)

If I’m sure that the subquery returns ~10 results, while table_a has thousands, can I somehow force table_a records to be looked up, and table_b result to be scanned? Mysql insists (according to EXPLAIN SELECT * FROM table_a...) to do ALL on table_a and index lookup on table_b.

Answer 1

Instead you can add index on field id and try INNER JOIN:

DELETE a
FROM table_a a 
     INNER JOIN table_b b
         ON a.id = b.id;