Re: Binary search in ScalarArrayOpExpr for OR'd constant arrays
От | Tomas Vondra |
---|---|
Тема | Re: Binary search in ScalarArrayOpExpr for OR'd constant arrays |
Дата | |
Msg-id | 20200428225233.t3xr4jhwiuxolmeh@development обсуждение исходный текст |
Ответ на | Re: Binary search in ScalarArrayOpExpr for OR'd constant arrays (James Coleman <jtc331@gmail.com>) |
Список | pgsql-hackers |
On Tue, Apr 28, 2020 at 06:22:20PM -0400, James Coleman wrote: >I cc'd Andres given his commit introduced simplehash, so I figured >he'd probably have a few pointers on when each one might be useful. > >On Tue, Apr 28, 2020 at 8:39 AM James Coleman <jtc331@gmail.com> wrote: >... >> > Any particular reasons to pick dynahash over simplehash? ISTM we're >> > using simplehash elsewhere in the executor (grouping, tidbitmap, ...), >> > while there are not many places using dynahash for simple short-lived >> > hash tables. Of course, that alone is a weak reason to insist on using >> > simplehash here, but I suppose there were reasons for not using dynahash >> > and we'll end up facing the same issues here. >> >> No particular reason; it wasn't clear to me that there was a reason to >> prefer one or the other (and I'm not acquainted with the codebase >> enough to know the differences), so I chose dynahash because it was >> easier to find examples to follow for implementation. > >Do you have any thoughts on what the trade-offs/use-cases etc. are for >dynahash versus simple hash? > >From reading the commit message in b30d3ea824c it seems like simple >hash is faster and optimized for CPU cache benefits. The comments at >the top of simplehash.h also discourage it's use in non >performance/space sensitive uses, but there isn't anything I can see >that explicitly tries to discuss when dynahash is useful, etc. > >Given the performance notes in that commit message, I thinking >switching to simple hash is worth it. > I recall doing some benchmarks for that patch, but it's so long I don't really remember the details. But in general, I agree simplehash is a bit more efficient in terms of CPU / caching. I think the changes required to switch from dynahash to simplehash are fairly small, so I think the best thing we can do is just try do some measurement and then decide. >But I also wonder if there might be some value in a README or comments >addition that would be a guide to what the various hash >implementations are useful for. If there's interest, I could try to >type something short up so that we have something to make the code >base a bit more discoverable. > I wouldn't object to that. Although maybe we should simply add some basic recommendations to the comments in dynahash/simplehash. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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