Question

select docid  from A  where  docid IN ( select distinct(docid) from B)

When I execute above query in mysql it takes 33 seconds, which is too long as per the size of data.

Below is the details of both tables.

   Table A :
   | docid       | int(11)  | NO   | PRI | NULL    |       |
   Total number of entries = 500 (all entries are unique)

   Table B:
   | docid       | int(11)  | YES  |     | NULL    |       |
   Total number of entries = 66508
   (number of unique entries are 500)

   mysql version : 5.2

If I execute only select docid from A it will take 0.00 seconds,
while select docid from B is taking 0.07 seconds.

Then why IN query with subquery takes 33 seconds? Am I doing something wrong?

Answer 1

MySQL doesn’t handle IN (subquery) well. It executes the inner query every single time the outer query is evaluated, rather than “remembering” the results.

Hence you are much better doing a join.

Other RDBMSes don’t do this btw.