pgsql: Fix misprocessing of equivalence classes involving record_eq().

Fix misprocessing of equivalence classes involving record_eq().

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

Branch
------
REL9_3_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bc711befd6ce24033175584f900e0cf43af81f5c

Modified Files
--------------
src/backend/optimizer/path/equivclass.c | 3 ++-
src/test/regress/expected/join.out | 30 ++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 20 ++++++++++++++++++++
3 files changed, 52 insertions(+), 1 deletion(-)