MYSQL Query Optimization: Avoiding ORs

So, you've probably heard that using ORs in your queries is heavy and should be avoided when possible. Here is a live example. We had the following query running on a medium sized table in MYSQL (around 200K rows):

SELECT count(*)  FROM interactions p
   WHERE (
       (p.employer = 69 AND p.flag1 = 1)
       OR
       (p.employee = 69 AND p.flag2 = 0)
       )

Let's think of interactions as a table holding the interaction between employers and employees, and this query should tell the user the total number of interactions where user number 69 is involved.

The query looked pretty innoccent on an idle MYSQL server taking around 200ms, but when the server got busy the query execution time was reaching 5-6 seconds.

Altough we did have indexes on interactions.employer and interactions. employee doing EXPLAIN showed that MYSQL was not using them.

After some digging and fiddling and trying we ended up with:

SELECT count(*) as count FROM (
   SELECT 1  FROM interactions p
       WHERE (p.employer = 69 AND p.flag1 = 1)
   UNION ALL
   SELECT 1  FROM privatemsg p
       WHERE (p.employee = 69 AND p.flag2 = 0)
) t2

This query took on the busy server less then 250 milliseconds and less then 80ms on the idle server. Doing EXPLAIN showed that MYSQL was doing two queries, and using the correct index on each one.

After some more digging we noticed that if we EXPLAIN the original query on the idle server the optimizer occasionally converts it to a UNION, but for some reason this did not always happen and in any case took about twice the time then the UNION query.

To sum up the results here is a simple table

Query type	Server activity	Query Time
Using OR	Idle	200ms
Using OR	Busy	5500ms
Using UNION	Idle	78ms
Using UNION	Busy	189ms

Conclusion: always consider alternatives to OR, but make sure you check them well against real-time examples.

A Further Note: When timing queries in MYSQL alway use the optimizer hint /*! SQL_NO_CACHE */ to make sure you are not getting results from the query cache (if you have one set up).