ef03b34550
This fixes an oversight in commit 11cad29c91524aac1d0b61e0ea0357398ab79bf8, which introduced MergeAppend plans. Before that happened, we never particularly cared about the sort ordering of scans of inheritance child relations, since appending their outputs together would destroy any ordering anyway. But now it's important to be able to match child relation sort orderings to those of the surrounding query. The original coding of add_child_rel_equivalences skipped ec_has_const EquivalenceClasses, on the originally-correct grounds that adding child expressions to them was useless. The effect of this is that when a parent variable is equated to a constant, we can't recognize that index columns on the equivalent child variables are not sort-significant; that is, we can't recognize that a child index on, say, (x, y) is able to generate output in "ORDER BY y" order when there is a clause "WHERE x = constant". Adding child expressions to the (x, constant) EquivalenceClass fixes this, without any downside that I can see other than a few more planner cycles expended on such queries. Per recent gripe from Robert McGehee. Back-patch to 9.1 where MergeAppend was introduced.
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