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62e0020ad4
canonicalize_ec_expression() is supposed to agree with coerce_type() as to whether a RelabelType should be inserted to make a subexpression be valid input for the operators of a given opclass. However, it did the wrong thing with named-composite-type inputs to record_eq(): it put in a RelabelType to RECORDOID, which the parser doesn't. In some cases this was harmless because all code paths involving a particular equivalence class did the same thing, but in other cases this would result in failing to recognize a composite-type expression as being a member of an equivalence class that it actually is a member of. The most obvious bad effect was to fail to recognize that an index on a composite column could provide the sort order needed for a mergejoin on that column, as reported by Teodor Sigaev. I think there might be other, subtler, cases that result in misoptimization. It also seems possible that an unwanted RelabelType would sometimes get into an emitted plan --- but because record_eq and friends don't examine the declared type of their input expressions, that would not create any visible problems. To fix, just treat RECORDOID as if it were a polymorphic type, which in some sense it is. We might want to consider formalizing that a bit more someday, but for the moment this seems to be the only place where an IsPolymorphicType() test ought to include RECORDOID as well. This has been broken for a long time, so back-patch to all supported branches. Discussion: https://postgr.es/m/a6b22369-e3bf-4d49-f59d-0c41d3551e81@sigaev.ru |
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