[gcarre]

"MySQL Partitioning. We decided against it because it seemed likely that it wouldn’t be much faster than our current solution."

What if you partition by HASH(user_id) instead of partitioning by month (http://dev.mysql.com/doc/refman/5.5/en/partitioning-hash.htm...)?

[lann]

(another Causes engineer here) HASH partitioning could make sense if our queries were primarily filtering on e.g. user_id, but that isn't the case here. Instead we are often querying against one or both of two columns: user_id and action_id. Partitioning by timestamp allows us to ignore partitions older than the user and/or action's creation dates.