pgsql: Don't lose partitioned table reltuples=0 after relhassubclass=f.

Don't lose partitioned table reltuples=0 after relhassubclass=f.

ANALYZE sets relhassubclass=f when a partitioned table no longer has
partitions.  An ANALYZE doing that proceeded to apply the inplace update
of pg_class.reltuples to the old pg_class tuple instead of the new
tuple, losing that reltuples=0 change if the ANALYZE committed.
Non-partitioning inheritance trees were unaffected.  Back-patch to v14,
where commit 375aed36ad83f0e021e9bdd3a0034c0c992c66dc introduced
maintenance of partitioned table pg_class.reltuples.

Reported by Alexander Lakhin.

Discussion: https://postgr.es/m/a295b499-dcab-6a99-c06e-01cf60593344@gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2b415e95a8a7ccb4fe9276e2ca453f4ae69466c1

Modified Files
--------------
src/backend/commands/analyze.c             |  7 ++++-
src/test/regress/expected/sanity_check.out |  2 ++
src/test/regress/expected/vacuum.out       | 47 ++++++++++++++++++++++++++++++
src/test/regress/sql/vacuum.sql            | 29 ++++++++++++++++++
4 files changed, 84 insertions(+), 1 deletion(-)