On 2017-06-01 18:41:20 +0530, Rafia Sabih wrote: > As per my understanding it looks like this increase in tuple queue > size is helping only gather-merge. Particularly, in the case where it > is enough stalling by master in gather-merge because it is maintaining > the sort-order. Like in q12 the index is unclustered and gather-merge > is just above parallel index scan, thus, it is likely that to maintain > the order the workers have to wait long for the in-sequence tuple is > attained by the master.
I wonder if there's some way we could make this problem a bit less bad. One underlying problem is that we don't know what the current boundary on each worker is, unless it returns a tuple. I.e. even if some worker is guaranteed to not return any further tuples below another worker's last tuple, gather-merge won't know about that until it finds another matching tuple. Perhaps, for some subsets, we could make the workers update that boundary without producing a tuple that gather will actually return? In the, probably reasonably common, case of having merge-joins below the gather, it shouldn't be very hard to do so. Imagine e.g. that every worker gets a "slot" in a dsm where it can point to a tuple (managed by dsa.c to deal with variable-length keys) that contains the current boundary. For a merge-join it'd not be troublesome to occasionally - although what constitutes that isn't easy, perhaps the master signals the worker? - put a new boundary tuple there, even if it doesn't find a match. It's probably harder for cases where most of the filtering happens far below the top-level worker node. - Andres -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers