On Tue, Mar 27, 2018 at 5:36 PM, Magnus Hagander <mag...@hagander.net> wrote: > On Tue, Mar 27, 2018 at 12:28 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> >> David Rowley <david.row...@2ndquadrant.com> writes: >> > On 27 March 2018 at 09:27, Tom Lane <t...@sss.pgh.pa.us> wrote: >> >> I do not think it is accidental that these aggregates are exactly the >> >> ones >> >> that do not have parallelism support today. Rather, that's because you >> >> just about always have an interest in the order in which the inputs get >> >> aggregated, which is something that parallel aggregation cannot >> >> support. >> >> > This very much reminds me of something that exists in the 8.4 release >> > notes: >> >> SELECT DISTINCT and UNION/INTERSECT/EXCEPT no longer always produce >> >> sorted output (Tom) >> >> That's a completely false analogy: we got a significant performance >> benefit for a significant fraction of users by supporting hashed >> aggregation. My argument here is that only a very tiny fraction of >> string_agg/array_agg users will not care about aggregation order, and thus >> I don't believe that this patch can help very many people. Against that, >> it's likely to hurt other people, by breaking their queries and forcing >> them to insert expensive explicit sorts to fix it. Even discounting the >> backwards-compatibility question, we don't normally adopt performance >> features for which it's unclear that the net gain over all users is >> positive. > > > I think you are quite wrong in claiming that only a tiny fraction of the > users are going to care. > > This may, and quite probably does, hold true for string_agg(), but not for > array_agg(). I see a lot of cases where people use that to load it into an > unordered array/hashmap/set/whatever on the client side, which looses > ordering *anyway*,and they would definitely benefit from it.
Agreed, I have seen lots of uses of array_agg where the order didn't matter. cheers andrew -- Andrew Dunstan https://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services