On Sat, Feb 18, 2017 at 6:43 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: > I think there is a value in supporting mark/restore position for any > node which produces sorted results, however, if you don't want to > support it, then I think we should update above comment in the code to > note this fact. Also, you might want to check other places to see if > any similar comment updates are required in case you don't want to > support mark/restore position for GatherMerge.
I don't think it makes sense to put mark/support restore into Gather Merge. Maybe somebody else will come up with a test that shows differently, but ISTM that with something like Sort it makes a ton of sense to support mark/restore because the Sort node itself can do it much more cheaply than would be possible with a separate Materialize node. If you added a separate Materialize node, the Sort node would be trying to throw away the exact same data that the Materialize node was trying to accumulate, which is silly. Here with Gather Merge there is no such thing happening; mark/restore support would require totally new code - probably we would end up shoving the same code that already exists in Materialize into Gather Merge as well. That doesn't seem like a good idea off-hand. A comment update is probably a good idea, though. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers