pgsql: Propagate sort instrumentation from workers back to leader.

Propagate sort instrumentation from workers back to leader.

Up until now, when parallel query was used, no details about the
sort method or space used by the workers were available; details
were shown only for any sorting done by the leader. Fix that.

Commit 1177ab1dabf72bafee8f19d904cee3a299f25892 forced the test case
added by commit 1f6d515a67ec98194c23a5db25660856c9aab944 to run
without parallelism; now that we have this infrastructure, allow
that again, with a little tweaking to make it pass with and without
force_parallel_mode.

Robert Haas and Tom Lane

Discussion: http://postgr.es/m/CA+Tgmoa2VBZW6S8AAXfhpHczb=Rf6RqQ2br+zJvEgwJ0uoD_tQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bf11e7ee2e3607bb67d25aec73aa53b2d7e9961b

Modified Files
--------------
src/backend/commands/explain.c | 57 +++++++++++-
src/backend/executor/execParallel.c | 155 ++++++++++++++++++--------------
src/backend/executor/nodeSort.c | 97 ++++++++++++++++++++
src/backend/utils/sort/tuplesort.c | 56 +++++++++---
src/include/executor/nodeSort.h | 7 ++
src/include/nodes/execnodes.h | 12 +++
src/include/utils/tuplesort.h | 34 ++++++-
src/test/regress/expected/subselect.out | 5 +-
src/test/regress/sql/subselect.sql | 6 +-
9 files changed, 342 insertions(+), 87 deletions(-)