Question

I found that the following query is causing some bottleneck (the execution time takes more than 40 seconds!)

DELETE FROM doctors 
WHERE (SELECT COUNT(loc_id) 
       FROM locations 
       WHERE locations.loc_doctor = doctors.doc_id) = 0 AND 
       doctors.doc_user = $myVar

I guess the problem lies in the (SELECT COUNT(loc_id) FROM locations WHERE locations.loc_doctor = doctors.doc_id) = 0 section, am I right?
Is there a way to improve it?

Answer 1

This should be a little bit faster:

DELETE FROM doctors WHERE doctors.doc_user = $myVar AND NOT EXISTS (SELECT 1 FROM locations WHERE locations.loc_doctor = doctors.doc_id LIMIT 1)

As your counting for 0 is actually a NOT EXISTS check. You should also consider an index for locations.loc_doctor column (if you don’t have one already).