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Fix planner error (or assert trap) with nested set operations.
As reported by Sean Johnston in bug #14614, since 9.6 the planner can fail due to trying to look up the referent of a Var with varno 0. This happens because we generate such Vars in generate_append_tlist, for lack of any better way to describe the output of a SetOp node. In typical situations nothing really cares about that, but given nested set-operation queries we will call estimate_num_groups on the output of the subquery, and that wants to know what a Var actually refers to. That logic used to look at subquery->targetList, but in commit 3fc6e2d I'd switched it to look at subroot->processed_tlist, ie the actual output of the subquery plan not the parser's idea of the result. It seemed like a good idea at the time :-(. As a band-aid fix, change it back. Really we ought to have an honest way of naming the outputs of SetOp steps, which suggests that it'd be a good idea for the parser to emit an RTE corresponding to each one. But that's a task for another day, and it certainly wouldn't yield a back-patchable fix. Report: https://postgr.es/m/20170407115808.25934.51866@wrigleys.postgresql.org
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