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find_computable_ec_member() had the wrong mental model of what its primary caller prepare_sort_from_pathkeys() would do with the selected EquivalenceClass member expression. We will not compute the EC expression in a plan node atop the one returning the passed-in targetlist; rather, the EC expression will be computed as an additional column of that targetlist. So any Var or quasi-Var used in the given tlist is also available to the EC expression. In simple cases this makes no difference because the given tlist is just a list of Vars or quasi-Vars --- but if we are considering an appendrel member produced by flattening a UNION ALL, the tlist may contain expressions, resulting in failure to match and a "could not find pathkey item to sort" error. To fix, we can flatten both the tlist and the EC members with pull_var_clause(), and then just check for subset-ness, so that the code is actually shorter than before. While this bug is quite old, the present patch only works back to v13. We could possibly make it work in v12 by back-patching parts of 375398244. On the whole though I don't like the risk/reward ratio of that idea. v12's final release is next month, meaning there would be no chance to correct matters if the patch causes a regression. Since this failure has escaped notice for 14 years, it's likely nobody will hit it in the field with v12. Per bug #18652 from Alexander Lakhin. Andrei Lepikhov and Tom Lane Discussion: https://postgr.es/m/18652-deaa782ebcca85d1@postgresql.org
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