Super slow SQL query when `WHERE` and `OR` are used together [duplicate] - mysql

Question

The following query takes mysql to execute almost 7 times longer than implementing the same using two separate queries, and avoiding OR on the WHERE statement. I prefer using a single query as I can sort and group everything.
Here is the problematic query:
EXPLAIN SELECT *
FROM `posts`
LEFT JOIN `teams_users`
ON (teams_users.team_id=posts.team_id
AND teams_users.user_id='7135')
WHERE (teams_users.status='1'
OR posts.user_id='7135');
Result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE posts ALL user_id NULL NULL NULL 169642
1 SIMPLE teams_users eq_ref PRIMARY PRIMARY 8 posts.team_id,const 1 Using where
Now if I do the following two queries instead, the aggregate execution time, as said, is shorter by 7 times:
EXPLAIN SELECT *
FROM `posts`
LEFT JOIN `teams_users`
ON (teams_users.team_id=posts.team_id
AND teams_users.user_id='7135')
WHERE (teams_users.status='1');
Result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE teams_users ref PRIMARY,status status 1 const 5822 Using where
1 SIMPLE posts ref team_id team_id 5 teams_users.team_id 9 Using where
and:
EXPLAIN SELECT *
FROM `posts`
LEFT JOIN `teams_users`
ON (teams_users.team_id=posts.team_id
AND teams_users.user_id='7135')
WHERE (posts.user_id='7135');
Result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE posts ref user_id user_id 4 const 142
1 SIMPLE teams_users eq_ref PRIMARY PRIMARY 8 posts.team_id,const 1
Obviously the amount of scanned rows is much lower on the two queries.
Why is the initial query slow?
Thanks.

Answer 1

Yes, OR is frequently a performance-killer. A common work-around is to do UNION. For your example:
SELECT *
FROM `posts`
LEFT JOIN `teams_users`
ON (teams_users.team_id=posts.team_id
AND teams_users.user_id='7135')
WHERE (teams_users.status='1')
UNION DISTINCT
SELECT *
FROM `posts`
LEFT JOIN `teams_users`
ON (teams_users.team_id=posts.team_id
AND teams_users.user_id='7135')
WHERE (posts.user_id='7135');
If you are sure there are not dups, change to the faster UNION ALL.
If you are not fishing for missing team_users rows, use JOIN instead of LEFT JOIN.
If you need ORDER BY, add some parens:
( SELECT ... )
UNION ...
( SELECT ... )
ORDER BY ...
Otherwise, the ORDER BY would apply only to the second SELECT. (If you also need 'pagination', see my blog .)
Please note that you might also need LIMIT in certain circumstances.

Answer 2

The queries without the OR clause are both sargable. That is, they both can be satisfied using indexes.
The query with the OR would be sargable if the MySQL query planner contained logic to figure out it can rewrite it as the UNION ALL of two queries. By the MySQL query planner doesn't (yet) have that kind of logic.
So, it does table scans to get the result set. Those are often very slow.