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For UNION ALL queries where a union child query contained a foreign table, if the targetlist of that query contained a constant, and the top-level query performed an ORDER BY which contained the column for the constant value, then postgres_fdw would find the EquivalenceMember with the Const and then try to produce an ORDER BY containing that Const. This caused problems with INT typed Consts as these could appear to be requests to order by an ordinal column position rather than the constant value. This could lead to either an error such as: ERROR: ORDER BY position <int const> is not in select list or worse, if the constant value is a valid column, then we could just sort by the wrong column altogether. Here we fix this issue by just not including these Consts in the ORDER BY clause. In passing, add a new section for testing ORDER BY in the postgres_fdw tests and move two existing tests which were misplaced in the WHERE clause testing section into it. Reported-by: Michał Kłeczek Reviewed-by: Ashutosh Bapat, Richard Guo Bug: #18381 Discussion: https://postgr.es/m/0714C8B8-8D82-4ABB-9F8D-A0C3657E7B6E%40kleczek.org Discussion: https://postgr.es/m/18381-137456acd168bf93%40postgresql.org Backpatch-through: 12, oldest supported version