Indexing 5 columns

Question

I have the following query:

SELECT * 
FROM table 
WHERE active = 1 
  AND deleted = 0 
  AND blocked = 0 
  AND created_at <= NOW() 
  AND last_action_at <= NOW() 
LIMIT 5000;

I have an index set on (deleted, active) , so it reduces me some rows, but out of 2,5mln records it still traverses 1,5mln.

I also tried adding the following indexes:

(created_at, last_action_at)
(active,deleted,blocked,created_at,last_action_at)

But the query doesn't use them, and uses the first one I mentioned instead. I also tried changing range conditions to the first position in WHERE clause, but it also didn't change anything.

What's wrong?

Answer 1

Nothing is wrong, it's just more efficient to use one index over another. In this case I would bet the index used is the clustered one.

Unless you are selecting a really tiny proportion of rows, no index will help unless it's the clustered one in which case it's really only used because it is actually the data, ie it's a slightly more efficiently bounded table scan.

You might also find that an index on a column that allows nulls can be excluded from a search if null is not excluded.

What's your expected final row count and your query plan?