Обсуждение: pgsql: Fix accumulation of parallel worker instrumentation.

Fix accumulation of parallel worker instrumentation.

When a Gather or Gather Merge node is started and stopped multiple
times, the old code wouldn't reset the shared state between executions,
potentially resulting in dramatically inflated instrumentation data
for nodes beneath it.  (The per-worker instrumentation ended up OK,
I think, but the overall totals were inflated.)

Report by hubert depesz lubaczewski.  Analysis and fix by Amit Kapila,
reviewed and tweaked a bit by me.

Discussion: http://postgr.es/m/20171127175631.GA405@depesz.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2c09a5c12a66087218c7f8cba269cd3de51b9b82

Modified Files
--------------
src/backend/executor/execParallel.c           | 51 ++++++++++++++++++++-------
src/test/regress/expected/select_parallel.out | 21 +++++++++++
src/test/regress/sql/select_parallel.sql      |  7 ++++
3 files changed, 66 insertions(+), 13 deletions(-)