From 76799fc89d2ca6f357c188daa4e1b38f0a59e360 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Mon, 17 Apr 2017 15:29:00 -0400 Subject: [PATCH] Always build a custom plan node's targetlist from the path's pathtarget. We were applying the use_physical_tlist optimization to all relation scan plans, even those implemented by custom scan providers. However, that's a bad idea for a couple of reasons. The custom provider might be unable to provide columns that it hadn't expected to be asked for (for example, the custom scan might depend on an index-only scan). Even more to the point, there's no good reason to suppose that this "optimization" is a win for a custom scan; whatever the custom provider is doing is likely not based on simply returning physical heap tuples. (As a counterexample, if the custom scan is an interface to a column store, demanding all columns would be a huge loss.) If it is a win, the custom provider could make that decision for itself and insert a suitable pathtarget into the path, anyway. Per discussion with Dmitry Ivanov. Back-patch to 9.5 where custom scan support was introduced. The argument that the custom provider can adjust the behavior by changing the pathtarget only applies to 9.6+, but on balance it seems more likely that use_physical_tlist will hurt custom scans than help them. Discussion: https://postgr.es/m/e29ddd30-8ef9-4da5-a50b-2bb7b8c7198d@postgrespro.ru --- src/backend/optimizer/plan/createplan.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c index 8179749ef125d..95e6eb7d2897a 100644 --- a/src/backend/optimizer/plan/createplan.c +++ b/src/backend/optimizer/plan/createplan.c @@ -799,6 +799,15 @@ use_physical_tlist(PlannerInfo *root, Path *path, int flags) if (rel->reloptkind != RELOPT_BASEREL) return false; + /* + * Also, don't do it to a CustomPath; the premise that we're extracting + * columns from a simple physical tuple is unlikely to hold for those. + * (When it does make sense, the custom path creator can set up the path's + * pathtarget that way.) + */ + if (IsA(path, CustomPath)) + return false; + /* * Can't do it if any system columns or whole-row Vars are requested. * (This could possibly be fixed but would take some fragile assumptions