On 2017-10-12 17:27:52 -0400, Robert Haas wrote: > On Thu, Oct 12, 2017 at 4:20 PM, Andres Freund <and...@anarazel.de> wrote: > >> In other words, it's not utterly fixed in stone --- we invented > >> --load-via-partition-root primarily to cope with circumstances that > >> could change hash values --- but we sure don't want to be changing it > >> with any regularity, or for a less-than-excellent reason. > > > > Yea, that's what I expected. It'd probably good for somebody to run > > smhasher or such on the output of the combine function (or even better, > > on both the 32 and 64 bit variants) in that case. > > Not sure how that test suite works exactly, but presumably the > characteristics in practice will depend the behavior of the hash > functions used as input the combine function - so the behavior could > be good for an (int, int) key but bad for a (text, date) key, or > whatever.
I don't think that's true, unless you have really bad hash functions on the the component hashes. A hash combine function can't really do anything about badly hashed input, what you want is that it doesn't *reduce* the quality of the hash by combining. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
