Question

In this sql:

    SELECT s.*,
           u.id,
           u.name 
      FROM shops s 
 LEFT JOIN users u ON u.id = s.user_id 
                   OR u.id = s.owner_user_id 
     WHERE s.status = 1

For some reason this query takes an amazing time. although id is the primary key. it seems especially after I added this part OR u.id=s.owner_user_id the query became slow. owner_user_id often is 0 only handful of times. But why would it take so long apparently scanning the whole table? The database table users is very long and big. I didn’t design it. this is for a client who subsequent programmers added too many fields. the table is 22k rows and dozens of fields.

*the names of the fields for demonstration only. actual names are different, so don’t ask me why I’m looking for owner_user_id (; I did solve the slowness by remove the “OR …” part and instead searching for the id in the loop if it is not 0. but I would like to know why this is happening and how to speedup that query as is.

Answer 1

You may be able to speed it up by using IN instead of the OR but that is minor.

SELECT u.id,
           u.name 
      FROM shops s
 LEFT JOIN users u ON u.id IN ( s.user_id, s.owner_user_id )
WHERE s.status = 1

Firstly, are there any indexes on this table? Mainly one on the user.id field or the s.user_id or s.owner_user_id?

However, I must ask why you need to use a LEFT JOIN instead of a regular join. The LEFT JOIN causes the matching of every row with every other one. And since I’m assuming the value / id should either be in the user_id or the owner_user_id field, and that there will always be a match, if that is the case then the use of a JOIN should speed the query up a bit.

And as Mitch said, 22k rows is tiny.