I'm trying to optimize some of the database queries in my Rails app and I have several that have got me stumped. They are all using an IN in the WHERE clause and are all doing full table scans even though an appropriate index appears to be in place. For example: SELECT `user_metrics`.* FROM `user_metrics` WHERE (`user_metrics`.user_id IN (N,N,N,N,N,N,N,N,N,N,N,N)) performs a full table scan and EX