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Re: [PATCHES] updated hash functions for postgresql v1

From: Kenneth Marshall <ktm(at)rice(dot)edu>
To: pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [PATCHES] updated hash functions for postgresql v1
Date: 2008-11-04 20:26:55
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Sorry about the delay for this update to the new hash
index implementation. I was trying to get the WAL logging
in place and forgot to post the actual patch. The WAL
for hash indexes will need to wait for 8.5, but I did
want to add back in the piece of the Bob Jenkins 2006
hash function that was stripped out of the initial
patch on application due to concerns about the randomness
of the resulting hash values. Here is a re-post of my
initial findings comparing the old/new Jenkins hash
from lookup2 and lookup3. I have added a third column
containing the results for the hash_any() resulting
from the attached patch as well as simple timing test
for a DB reindex both before and after patching.

Also attached is a simple documentation patch updating
the note attached to the hash index description.


I have finally had a chance to do some investigation on
the performance of the old hash mix() function versus
the updated mix()/final() in the new hash function. Here
is a table of my current results for both the old and the
new hash function. In this case cracklib refers to the
cracklib-dict containing 1648379 unique words massaged
in various ways to generate input strings for the hash
functions. The result is the number of collisions in the
hash values generated.

hash input                            old    new  newv2
----------                            ---    ---  -----
cracklib                              338    316  338
cracklib x 2 (i.e. clibclib)          305    319  300
cracklib x 3 (clibclibclib)           323    329  315
cracklib x 10                         302    310  329
cracklib x 100                        350    335  298
cracklib x 1000                       314    309  315
cracklib x 100 truncated to char(100) 311    327  320

uint32 from 1-1648379                 309    319  347
(uint32 1-1948379)*256                309    314  304
(uint32 1-1948379)*16                 310    314  324
"a"uint32 (i.e. a00001,a0002...)      320    321  312

uint32uint32 (i.e. uint64)            321    287  309

The different result columns are old = Jenkins 1996 hash
function(lookup2.c), new = Jenkins 2006 hash function
(lookup3.c), and newv2 = adaptation of current hash_any()
to incorporate the separate mix()/final() functions. As
you can see from the results, spliting the mix() and final()
apart does not result in any perceptible loss of randomness
in the hash assignment. I also ran a crude timing for a
reindex of the following database:

CREATE TABLE dict (word text);
CREATE INDEX wordhash ON dict USING hash (word);
INSERT INTO dict (word) VALUES('s;lhkjdpyoijxfg;lktjgh;sdlhkjo');
INSERT INTO dict (SELECT MAX(word)||MAX(word) FROM dict);
... (21 times)


The average time to reindex the table using our current
hash_any() without the separate mix()/final() was 1696ms
and 1482ms with the separate mix()/final() stages giving
almost 13% better performance for this stupid metric.


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Subject: Re: [PATCHES] updated hash functions for postgresql v1
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