Re: Memory-Bounded Hash Aggregation
От | Tomas Vondra |
---|---|
Тема | Re: Memory-Bounded Hash Aggregation |
Дата | |
Msg-id | 20190712065955.pm2rnk2hxa3qqawf@development обсуждение исходный текст |
Ответ на | Re: Memory-Bounded Hash Aggregation (Jeff Davis <pgsql@j-davis.com>) |
Список | pgsql-hackers |
On Thu, Jul 11, 2019 at 06:06:33PM -0700, Jeff Davis wrote: >On Thu, 2019-07-11 at 17:55 +0200, Tomas Vondra wrote: >> Makes sense. I haven't thought about how the hybrid approach would be >> implemented very much, so I can't quite judge how complicated would >> it be >> to extend "approach 1" later. But if you think it's a sensible first >> step, >> I trust you. And I certainly agree we need something to compare the >> other >> approaches against. > >Is this a duplicate of your previous email? > Yes. I don't know how I managed to send it again. Sorry. >I'm slightly confused but I will use the opportunity to put out another >WIP patch. The patch could use a few rounds of cleanup and quality >work, but the funcionality is there and the performance seems >reasonable. > >I rebased on master and fixed a few bugs, and most importantly, added >tests. > >It seems to be working with grouping sets fine. It will take a little >longer to get good performance numbers, but even for group size of one, >I'm seeing HashAgg get close to Sort+Group in some cases. > Nice! That's a very nice progress! >You are right that the missed lookups appear to be costly, at least >when the data all fits in system memory. I think it's the cache misses, >because sometimes reducing work_mem improves performance. I'll try >tuning the number of buckets for the hash table and see if that helps. >If not, then the performance still seems pretty good to me. > >Of course, HashAgg can beat sort for larger group sizes, but I'll try >to gather some more data on the cross-over point. > Yes, makes sense. I think it's acceptable as long as we consider this during costing (when we know in advance we'll need this) or treat it to be emergency measure. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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