Is EXISTS more efficient than COUNT(*)>0?

Question

I\'m using MySQL 5.1, and I have a query that\'s roughly of the form:

select count(*) from mytable where a = \"foo\" and b = \"bar\";

In my

Answer 1

The most reliable way is probably LIMIT 1, but that's not the point.

Provided you have an index like CREATE INDEX mytable_index_a_b ON mytable (a,b), MySQL should be smart enough to return the count from the index and not touch any rows at all. The benefit of LIMIT 1 is probably negligible.

If you don't have an index on (a,b), then performance will be terrible. LIMIT 1 may make it significantly less terrible, but it'll still be terrible.