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to simplify constant expressions and expand SubLink nodes into SubPlans is done in a separate routine subquery_planner() that calls union_planner(). We formerly did most of this work in query_planner(), but that's the wrong place because it may never see the real targetlist. Splitting union_planner into two routines also allows us to avoid redundant work when union_planner is invoked recursively for UNION and inheritance cases. Upshot is that it is now possible to do something like select float8(count(*)) / (select count(*) from int4_tbl) from int4_tbl group by f1; which has never worked before.