As of 6.1.1 TupleBPS::sendJobs() sends out JobList.MaxOutstandingRequests(20 extents of work by default) * 1024 Primitive Jobs simultaneously. By default it worths 20 extents of columnar data.
In case of queries like SELECT * FROM table_with_500_columns LIMIT 1 this overwhelms PPs with needless work and spoils the buffer cache. The optimizer must propagate the LIMIT into TupleBPS so that it issues a limited number of outstanding Primitive jobs for queries where it is semantically possible.