bf2d0462cd
Commit aa09cd242 changed a condition in find_em_expr_for_rel from being a bms_equal comparison of relids to bms_is_subset, in order to support order by clauses on foreign joins. But this also allows through the degenerate case of expressions with no Vars at all (and hence empty relids), including integer constants which will be parsed unexpectedly on the remote (viz. "ERROR: ORDER BY position 0 is not in select list" as in the bug report). Repair by adding an additional !bms_is_empty test. Backpatch through to 9.6 where the aforementioned change was made. Per bug #15352 from Maksym Boguk; analysis and patch by me. Discussion: https://postgr.es/m/153518420278.1478.14875560810251994661@wrigleys.postgresql.org
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