[COMMITTERS] pgsql: Code review for nodeGatherMerge.c.

Code review for nodeGatherMerge.c.

Comment the fields of GatherMergeState, and organize them a bit more
sensibly.  Comment GMReaderTupleBuffer more usefully too.  Improve
assorted other comments that were obsolete or just not very good English.

Get rid of the use of a GMReaderTupleBuffer for the leader process;
that was confusing, since only the "done" field was used, and that
in a way redundant with need_to_scan_locally.

In gather_merge_init, avoid calling load_tuple_array for
already-known-exhausted workers.  I'm not sure if there's a live bug there,
but the case is unlikely to be well tested due to timing considerations.

Remove some useless code, such as duplicating the tts_isempty test done by
TupIsNull.

Remove useless initialization of ps.qual, replacing that with an assertion
that we have no qual to check.  (If we did, the code would fail to check
it.)

Avoid applying heap_copytuple to a null tuple.  While that fails to crash,
it's confusing and it makes the code less legible not more so IMO.

Propagate a couple of these changes into nodeGather.c, as well.

Back-patch to v10, partly because of the possibility that the
gather_merge_init change is fixing a live bug, but mostly to keep
the branches in sync to ease future bug fixes.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/cb8e015b948d14d08b486ae1b2de879a0cc827d7

Modified Files
--------------
src/backend/executor/nodeGather.c      |  21 ++--
src/backend/executor/nodeGatherMerge.c | 185 ++++++++++++++++++---------------
src/include/nodes/execnodes.h          |  42 ++++----
3 files changed, 136 insertions(+), 112 deletions(-)