From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
Cc: | Greg Stark <stark(at)mit(dot)edu>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Combining hash values |
Date: | 2016-08-01 15:27:15 |
Message-ID: | 6883.1470065235@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)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.
regards, tom lane
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