I have a large database with login and level columns. All queries were slow before I created indexes on these columns:
CREATE INDEX users_login_index on users(login);
CREATE INDEX users_level_index on users(level);
Now I can quickly find a user by login (it's unique) or many users by level. This query is fast:
SELECT * FROM users WHERE login='somelogin123';
But this query is very slow:
SELECT * FROM users WHERE login='somelogin123' AND level=1;
Why the query becomes slow after adding an additional condition which is also indexed? How to solve my problem?