# :case_sensitive => false
SELECT `users`.id FROM `users` 
 WHERE (LOWER(`users`.`email`) = BINARY 'test@example.com') LIMIT 1
 
# :case_sensitive => true
SELECT `users`.id FROM `users` 
 WHERE (`users`.`email` = BINARY 'test@example.com') LIMIT 1
 
# without :case_sensitive option
SELECT `users`.id FROM `users` 
 WHERE (`users`.`email` = BINARY 'test@example.com') LIMIT 1

Rails always uses the BINARY keyword in the queries it issues to make the check case sensitive, regardless of the collation. That’s why I really need :case_sensitive => false for a case insensitive check, which uses the horribly inefficient query.

So unfortunately, this custom implementation really is needed to make sure the index on the field is used.

BTW: I changed a :case_sensitive => true into :case_sensitive => false. Thanks for spotting the mistake!