pgsql: Push tuple limits through Gather and Gather Merge.

Push tuple limits through Gather and Gather Merge.

If we only need, say, 10 tuples in total, then we certainly don't need
more than 10 tuples from any single process. Pushing down the limit
lets workers exit early when possible. For Gather Merge, there is
an additional benefit: a Sort immediately below the Gather Merge can
be done as a bounded sort if there is an applicable limit.

Robert Haas and Tom Lane

Discussion: http://postgr.es/m/CA+TgmoYa3QKKrLj5rX7UvGqhH73G1Li4B-EKxrmASaca2tFu9Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3452dc5240da43e833118484e1e9b4894d04431c

Modified Files
--------------
src/backend/executor/execParallel.c | 54 +++++++++---
src/backend/executor/execProcnode.c | 121 ++++++++++++++++++++++++++
src/backend/executor/nodeGather.c | 4 +-
src/backend/executor/nodeGatherMerge.c | 4 +-
src/backend/executor/nodeLimit.c | 98 ++++-----------------
src/include/executor/execParallel.h | 2 +-
src/include/executor/executor.h | 1 +
src/include/nodes/execnodes.h | 2 +
src/test/regress/expected/select_parallel.out | 24 ++++-
src/test/regress/sql/select_parallel.sql | 9 +-
10 files changed, 222 insertions(+), 97 deletions(-)