pgsql: Compare collations before merging UNION operations.

Compare collations before merging UNION operations.

In the dim past we figured it was okay to ignore collations
when combining UNION set-operation nodes into a single N-way
UNION operation. I believe that was fine at the time, but
it stopped being fine when we added nondeterministic collations:
the semantics of distinct-ness are affected by those. v17 made
it even less fine by allowing per-child sorting operations to
be merged via MergeAppend, although I think we accidentally
avoided any live bug from that.

Add a check that collations match before deciding that two
UNION nodes are equivalent. I also failed to resist the
temptation to comment plan_union_children() a little better.

Back-patch to all supported branches (v13 now), since they
all have nondeterministic collations.

Discussion: https://postgr.es/m/3605568.1731970579@sss.pgh.pa.us

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f286f64bcf01106fc08ea1c14252ae2c26b7d189

Modified Files
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src/backend/optimizer/prep/prepunion.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)