On Mon, Apr 20, 2015 at 10:08 PM, Robert Haas <robertmh...@gmail.com> wrote: > > On Tue, Apr 7, 2015 at 11:58 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: > > One disadvantage of retaining parallel-paths could be that it can > > increase the number of combinations planner might need to evaluate > > during planning (in particular during join path evaluation) unless we > > do some special handling to avoid evaluation of such combinations. > > Yes, that's true. But the overhead might not be very much. In the > common case, many baserels and joinrels will have no parallel paths > because the non-parallel paths is known to be better anyway. Also, if > parallelism does seem to be winning, we're probably planning a query > that involves accessing a fair amount of data,
Am I understanding right that by above you mean to say that retain the parallel and non-parallel path only if parallel-path wins over non-parallel path? If yes, then I am able to understand the advantage of retaining both parallel and non-parallel paths, else could you explain some more why you think it is advantageous to retain parallel-path even when it losses to serial path in the beginning? With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com