• Tom Lane's avatar
    Fix misprocessing of equivalence classes involving record_eq(). · a11b3bd3
    Tom Lane authored
    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
    a11b3bd3
join.sql 73.6 KB