Re: [PATCHES] updated hash functions for postgresql v1
От | Jeff Davis |
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Тема | Re: [PATCHES] updated hash functions for postgresql v1 |
Дата | |
Msg-id | 1231531455.25019.75.camel@jdavis обсуждение исходный текст |
Ответ на | Re: [PATCHES] updated hash functions for postgresql v1 (Kenneth Marshall <ktm@rice.edu>) |
Ответы |
Re: [PATCHES] updated hash functions for postgresql v1
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Список | pgsql-hackers |
On Mon, 2008-12-22 at 13:47 -0600, Kenneth Marshall wrote: > Dear PostgreSQL developers, > > I am re-sending this to keep this last change to the > internal hash function on the radar. > Hi Ken, A few comments: 1. New patch with very minor changes attached. 2. I reverted the change you made to indices.sgml. We still don't use WAL for hash indexes, and in my opinion we should continue to discourage their use until we do use WAL. We can add back in the comment that hash indexes are suitable for large keys if we have some results to show that. 3. There was a regression test failure in union.sql because the ordering of the results was different. I updated the regression test. 4. Hash functions affect a lot more than hash indexes, so I ran through a variety of tests that use a HashAggregate plan. Test setup and results are attached. These results show no difference between the old and the new code (about 0.1% better). 5. The hash index build time shows some improvement. The new code won in every instance in which a there were a lot of duplicates in the table (100 distinct values, 50K of each) by around 5%. The new code appeared to be the same or slightly worse in the case of hash index builds with few duplicates (1000000 distinct values, 5 of each). The difference was about 1% worse, which is probably just noise. Note: I'm no expert on hash functions. Take all of my tests with a grain of salt. I would feel a little better if I saw at least one test that showed better performance of the new code on a reasonable-looking distribution of data. The hash index build that you showed only took a second or two -- it would be nice to see a test that lasted at least a minute. Regards, Jeff Davis
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