Re: Default setting for enable_hashagg_disk
От | Tomas Vondra |
---|---|
Тема | Re: Default setting for enable_hashagg_disk |
Дата | |
Msg-id | 20200625231831.weorpgrjrlbtcv6d@development обсуждение исходный текст |
Ответ на | Re: Default setting for enable_hashagg_disk (Andres Freund <andres@anarazel.de>) |
Список | pgsql-hackers |
On Thu, Jun 25, 2020 at 11:16:23AM -0700, Andres Freund wrote: >Hi, > >On 2020-06-25 10:44:42 -0700, Jeff Davis wrote: >> There are only two possible paths: HashAgg and Sort+Group, and we need >> to pick one. If the planner expects one to spill, it is likely to >> expect the other to spill. If one spills in the executor, then the >> other is likely to spill, too. (I'm ignoring the case with a lot of >> tuples and few groups because that doesn't seem relevant.) > >There's also ordered index scan + Group. Which will often be vastly >better than Sort+Group, but still slower than HashAgg. > > >> Imagine that there was only one path available to choose. Would you >> suggest the same thing, that unexpected spills can exceed work_mem but >> expected spills can't? > >I'm not saying what I propose is perfect, but I've yet to hear a better >proposal. Given that there *are* different ways to implement >aggregation, and that we use expected costs to choose, I think the >assumed costs are relevant. > I share Jeff's opinion that this is quite counter-intuitive and we'll have a hard time explaining it to users. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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