Re: [PATCH] Incremental sort (was: PoC: Partial sort)
От | Tomas Vondra |
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Тема | Re: [PATCH] Incremental sort (was: PoC: Partial sort) |
Дата | |
Msg-id | 20190916103244.eqkifumgacmo6hmc@development обсуждение исходный текст |
Ответ на | Re: [PATCH] Incremental sort (was: PoC: Partial sort) (James Coleman <jtc331@gmail.com>) |
Ответы |
Re: [PATCH] Incremental sort (was: PoC: Partial sort)
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Список | pgsql-hackers |
On Sun, Sep 15, 2019 at 09:33:33PM -0400, James Coleman wrote: >On Sat, Jul 20, 2019 at 11:25 AM Tomas Vondra ><tomas.vondra@2ndquadrant.com> wrote: >> >> ... >> >> >> >> I think this may be a thinko, as this plan demonstrates - but I'm not >> >> sure about it. I wonder if this might be penalizing some other types of >> >> plans (essentially anything with limit + gather). >> >> >> >> Attached is a WIP patch fixing this by considering both startup and >> >> total cost (by calling compare_path_costs_fuzzily). >> > >> >It seems to me that this is likely a bug, and not just a changed >> >needed for this. Do you think it's better addressed in a separate >> >thread? Or retain it as part of this patch for now (and possibly break >> >it out later)? On the other hand, it's entirely possible that someone >> >more familiar with parallel plan limitations could explain why the >> >above comment holds true. That makes me lean towards asking in a new >> >thread. >> > >> >> Maybe. I think creating a separate thread would be useful, provided we >> manage to demonstrate the issue without an incremental sort. > >I did some more thinking about this, and I can't currently come up >with a way to reproduce this issue outside of this patch. It doesn't >seem reasonable to me to assume that there's anything inherent about >this patch that means it's the only way we can end up with a partial >path with a low startup cost we'd want to prefer. > >Part of me wants to pull it over to a separate thread just to get >additional feedback, but I'm not sure how useful that is given we >don't currently have an example case outside of this patch. > Hmm, I see. While I initially suggested to start a separate thread only if we have example not involving an incremental sort, that's probably not a hard requirement. I think it's fine to start a thead briefly explaining the issue, and pointing to incremental sort thread for actual example. > >One thing to note though: the current patch does not also modify >add_partial_path_precheck which also does not take into account >startup cost, so we probably need to update that for completeness's >sake. > Good point. It does indeed seem to make the same assumption about only comparing total cost before calling add_path_precheck. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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