On Mon, Aug 1, 2016 at 11:27 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Dean Rasheed <dean.a.rash...@gmail.com> writes: >> On that subject, while looking at hashfunc.c, I spotted that >> hashint8() has a very obvious deficiency, which causes disastrous >> performance with certain inputs: > > Well, if you're trying to squeeze 64 bits into a 32-bit result, there > are always going to be collisions somewhere. > >> I'd suggest using hash_uint32() for values that fit in a 32-bit >> integer and hash_any() otherwise. > > Perhaps, but this'd break existing hash indexes. That might not be > a fatal objection, but if we're going to put users through that > I'd like to think a little bigger in terms of the benefits we get. > I've thought for some time that we needed to move to 64-bit hash function > results, because the size of problem that's reasonable to use a hash join > or hash aggregation for keeps increasing. Maybe we should do that and fix > hashint8 as a side effect.
Well, considering that Amit is working on makes hash indexes WAL-logged in v10, this seems like an awfully good time to get any breakage we want to do out of the way. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company  https://www.postgresql.org/message-id/caa4ek1lfzczyxloxs874ad0+s-zm60u9bwcyiuzx9mhz-kc...@mail.gmail.com -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers