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Commit6c73b39

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Always build a custom plan node's targetlist from the path's pathtarget.
We were applying the use_physical_tlist optimization to all relationscan plans, even those implemented by custom scan providers. However,that's a bad idea for a couple of reasons. The custom provider mightbe 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 provideris 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 customprovider could make that decision for itself and insert a suitablepathtarget into the path, anyway.Per discussion with Dmitry Ivanov. Back-patch to 9.5 where custom scansupport was introduced. The argument that the custom provider can adjustthe behavior by changing the pathtarget only applies to 9.6+, but onbalance it seems more likely that use_physical_tlist will hurt customscans than help them.Discussion:https://postgr.es/m/e29ddd30-8ef9-4da5-a50b-2bb7b8c7198d@postgrespro.ru
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‎src/backend/optimizer/plan/createplan.c

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@@ -761,6 +761,15 @@ use_physical_tlist(PlannerInfo *root, Path *path, int flags)
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if (rel->reloptkind!=RELOPT_BASEREL)
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return false;
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/*
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* Also, don't do it to a CustomPath; the premise that we're extracting
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* columns from a simple physical tuple is unlikely to hold for those.
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* (When it does make sense, the custom path creator can set up the path's
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* pathtarget that way.)
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*/
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if (IsA(path,CustomPath))
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return false;
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/*
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* Can't do it if any system columns or whole-row Vars are requested.
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* (This could possibly be fixed but would take some fragile assumptions

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