pgsql: Fix confusion about havingQual vs hasHavingQual in planner.

Fix confusion about havingQual vs hasHavingQual in planner.

Preprocessing of the HAVING clause will reduce havingQual to NIL
if the clause is constant-TRUE. This is one case where that
convention is rather unfortunate, because "HAVING TRUE" is not at all
the same as not having any HAVING clause at all. (Per the SQL spec,
it still forces the query to be grouped.) The planner deals with this
by having a boolean hasHavingQual that records whether havingQual was
originally nonempty; places that just want to check whether HAVING
was specified are supposed to consult that.

I found three places that got that wrong. Fortunately, these could
only affect cost estimates not correctness. It'd be hard even
to demonstrate the errors; for example, the one in allpaths.c would
only matter in a query that has HAVING TRUE but no GROUP BY and no
aggregates, which would require a completely variable-free SELECT
list, making the case probably of only academic interest. Hence,
while these are worth fixing before someone copies the incorrect
coding somewhere more critical, they don't seem worth back-patching.
I didn't bother trying to devise regression tests, either.

Discussion: https://postgr.es/m/2503888.1666042643@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8bf66dedd88673f94122c3f3337e7d58e561ef4f

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 4 ++--
src/backend/optimizer/path/allpaths.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)