b28ffd0fcc
In commit 57664ed25e5dea117158a2e663c29e60b3546e1c, I made the planner wrap non-simple-variable outputs of appendrel children (IOW, child SELECTs of UNION ALL subqueries) inside PlaceHolderVars, in order to solve some issues with EquivalenceClass processing. However, this means that any upper-level WHERE clauses mentioning such outputs will now contain PlaceHolderVars after they're pushed down into the appendrel child, and that prevents indxpath.c from recognizing that they could be matched to index expressions. To fix, add explicit stripping of PlaceHolderVars from index operands, same as we have long done for RelabelType nodes. Add a regression test covering both this and the plain-UNION case (which is a totally different code path, but should also be able to do it). Per bug #6416 from Matteo Beccati. Back-patch to 9.1, same as the previous change.
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