pgsql: Don't absorb child pathkeys into dummy AppendPaths for set-ops.

Don't absorb child pathkeys into dummy AppendPaths for set-ops.

create_append_path() has a rather questionable habit of overriding
the caller-supplied pathkeys when it sees that there is a single
child path, and applying the child's pathkeys instead. In most cases
we can get away with that, but it does not work for set-operation
AppendPaths. In set-operation nests, the append's result tlist will
contain "varno 0" Vars, which won't match the child's pathkeys,
leading to failure in create_plan().

We didn't have this problem before 03d40e4b5 allowed eliding
provably-empty child nodes of set-ops; there would never have been
a case with only one surviving child node, so create_append_path()
would always have accepted the specified NIL pathkeys.

As a band-aid fix, force the generated AppendPath's pathkeys to NIL
even if create_append_path() did something else, thus restoring the
status quo ante. This is demonstrably necessary for two of
prepunion.c's three calls; I did it at the third too, although
probably the pathkeys would already be NIL there.

There have been reasons to want to get rid of the "varno 0" hack
for a long time, and this is another one. But that will require
significant surgery in prepunion.c, and likely some changes in
parsetree representation, so we can't tackle it for v19. Hence,
we need a band-aid.

Bug: #19742
Reported-by: Junwen AN <feasiblechart(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Co-authored-by: shihao zhong <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/19742-dc403ca277cad1d3@postgresql.org
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2509f3c6b529a36ae7f2a53967af296afdc19f78

Modified Files
--------------
src/backend/optimizer/prep/prepunion.c | 16 +++++++
src/test/regress/expected/union.out | 80 +++++++++++++++++++++++++++++++++-
src/test/regress/sql/union.sql | 29 +++++++++++-
3 files changed, 123 insertions(+), 2 deletions(-)