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Fix several hash functions that were taking chintzy shortcuts instead of

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delivering a well-randomized hash value.  I got religion on this after
observing that performance of multi-batch hash join degrades terribly if the
higher-order bits of hash values aren't random, as indeed was true for say
hashes of small integer values.  It's now expected and documented that hash
functions should use hash_any or some comparable method to ensure that all
bits of their output are about equally random.

initdb forced because this change invalidates existing hash indexes.  For the
same reason, this isn't back-patchable; the hash join performance problem
will get a band-aid fix in the back branches.
This commit is contained in:
Tom Lane
2007-06-01 15:33:19 +00:00
parent 397d00af8f
commit 1f559b7d3a
5 changed files with 67 additions and 41 deletions
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3
src/include/access/hash.h
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@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/access/hash.h,v 1.81 2007/05/30 20:12:02 tgl Exp $
* $PostgreSQL: pgsql/src/include/access/hash.h,v 1.82 2007/06/01 15:33:18 tgl Exp $
*
* NOTES
* modeled after Margo Seltzer's hash implementation for unix.
@@ -265,6 +265,7 @@ extern Datum hashname(PG_FUNCTION_ARGS);
extern Datum hashtext(PG_FUNCTION_ARGS);
extern Datum hashvarlena(PG_FUNCTION_ARGS);
extern Datum hash_any(register const unsigned char *k, register int keylen);
extern Datum hash_uint32(uint32 k);
/* private routines */