It's about time we should fix this - right now ActiveRecord::ConnectionAdapters::MysqlAdapter has this code block:

def case_sensitive_equality_operator
  "= BINARY"
end

which renders validates_uniqueness_of prohibitively slow. With 7M records, it takes 3+ seconds in our case. It really should be:

def case_sensitive_equality_operator
  "="
end

That way, we benefit from MySQL's collation for case sensitivity in the index (like utf8_unicode_ci or utf8_bin), which is FAST. Or we could add :case_sensitive => nil as a new option particularly for that case.

It's a known problem for a long time actually, and patches have been around:

• http://grosser.it/2009/12/11/validates_uniqness_of-mysql-slow/
• http://techblog.floorplanner.com/2009/11/17/case-insensitive-validates_uniqueness_of-slowness/