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If an EquivalenceClass member expression includes variables from multiple appendrels, then instead of producing one substituted expression per child relation as intended, we'd create additional child expressions for combinations of children of different appendrels. This happened because the child expressions generated while considering the first appendrel were taken as sources during substitution of the second appendrel, and so on. The extra expressions are useless, and are harmless unless there are too many of them --- but if you have several appendrels with a thousand or so members each, it gets bad fast. To fix, consider only original (non-em_is_child) EC members as candidates to be expanded. This requires the ability to substitute directly from a top parent relation's Vars to those of an indirect descendant relation, but we already have that in adjust_appendrel_attrs_multilevel(). Per bug #15847 from Feike Steenbergen. This is a longstanding misbehavior, but it's only worth worrying about when there are more appendrel children than we've historically considered wise to use. So I'm not going to take the risk of back-patching this. Discussion: https://postgr.es/m/15847-ea3734094bf8ae61@postgresql.org
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