How to make an SQL Query process faster using NOT IN

Question

I have this SQL Query:

SELECT sequence, ticketnumber FROM tickets t
WHERE t.ticketnumber NOT IN (SELECT ticketnumber FROM ticket_updates)

it is querying thousands of rows in both tables - how can i make it run a little faster? Is there an alternative?

Answer 1

MySQL doesn't optimize this type of subquery well. It will keep running the second SELECT for every row in the outer query. Use a LEFT JOIN instead:

SELECT sequence, ticketnumber FROM tickets t LEFT JOIN ticket_updates tu ON (tu.ticketnumber = t.ticketnumber) WHERE tu.ticketnumber IS NULL;

Answer 2

Try this:

SELECT sequence, ticketnumber FROM tickets t
WHERE NOT EXISTS (SELECT u.ticketnumber FROM ticket_updates u WHERE u.ticketnumber = t.ticketnumber)

The NOT EXISTS query is optimized for this situation (just seeing if a row does/doesn't exist). It's not the only option and other queries will evaluate to the same operation, but this is what I would choose because its clear what is being asked.

Also, for this particular query (the NOT EXISTS ), indexing ticket_updates.ticketnumber would also yield a performance increase since the lookup speed would probably go from O(n) to O(log(n)) or O(1) (complete guess here...please correct me if I am utterly wrong).