Re: Question about some changes in 11.3
От | Tom Lane |
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Тема | Re: Question about some changes in 11.3 |
Дата | |
Msg-id | 8099.1560377694@sss.pgh.pa.us обсуждение исходный текст |
Ответ на | Re: Question about some changes in 11.3 (Mat Arye <mat@timescale.com>) |
Список | pgsql-hackers |
Mat Arye <mat@timescale.com> writes: > On Mon, Jun 3, 2019 at 4:07 PM Tom Lane <tgl@sss.pgh.pa.us> wrote: >> Hm. I'd say this was already broken by the invention of >> apply_scanjoin_target_to_paths; perhaps 11-before-11.3 managed to >> still work for you, but it's not hard to envision applications of >> set_rel_pathlist_hook for which it would not have worked. The contract >> for set_rel_pathlist_hook is supposed to be that it gets to editorialize >> on all normal (non-Gather) paths created by the core code, and that's >> no longer the case now that apply_scanjoin_target_to_paths can add more. >> ... >> I wonder whether we could fix matters by postponing the >> set_rel_pathlist_hook call till later in the same cases where >> we postpone generate_gather_paths, ie, it's the only baserel. > Is it really guaranteed that `apply_scanjoin_target_to_paths` will not be > called in other cases? Well, apply_scanjoin_target_to_paths is called in *all* cases. It only throws away the original paths for partitioned rels, though. I spent some more time looking at this, and I am afraid that my idea of postponing set_rel_pathlist_hook into apply_scanjoin_target_to_paths isn't going to work: there is not anyplace in that function where we could call the hook without the API being noticeably different from what it is at the current call site. In particular, if we try to call it down near the end so that it still has the property of being able to remove any core-generated path, then there's a *big* difference for queries involving SRFs: we've already plastered ProjectSetPath(s) atop the original paths, and any user of the hook would have to know to do likewise for freshly-generated paths. That would certainly break existing hook users. I'm inclined to think that the safest course is to leave set_rel_pathlist_hook as it stands, and invent a new hook that is called in apply_scanjoin_target_to_paths just before the generate_gather_paths call. (Or, perhaps, just after that --- but the precedent of set_rel_pathlist_hook suggests that "before" is more useful.) For your use-case you'd have to get into both hooks, and they'd both have to know that if they're dealing with a partitioned baserel that is the only baserel in the query, the new hook is where to generate paths rather than the old hook. Maybe it'd be worth having the core code export some simple test function for that, rather than having the details of those semantics be wired into various extensions. I think it'd be all right to put a patch done that way into the v11 branch. It would not make anything any worse for code that uses set_rel_pathlist_hook and is OK with the v11 behavior. Code that needs to use the new hook would fail to load into 11-before-11.whatever, but that's probably better than loading and then doing the wrong thing. regards, tom lane
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