InnoDB forum DB design and Composite Primary Key (Cluster Primary Key)
You can and will be better served by removing created_by from your primary key and creating a separate index for it. 

This will allow those queries which use created_by in the where clause (e.g. where created_by = ?), to be able to use that index to service the query. 

There may be a slight, and probably not noticeable, performance hit from managing the primary key and the index on an insert. However this price is nothing to pay for not doing full table scans when executing such queries needing it to be be the primary search in the where clause.