pgsql: Allow DML commands that create tables to use parallel query.

Allow DML commands that create tables to use parallel query.

Haribabu Kommi, reviewed by Dilip Kumar and Rafia Sabih. Various
cosmetic changes by me to explain why this appears to be safe but
allowing inserts in parallel mode in general wouldn't be. Also, I
removed the REFRESH MATERIALIZED VIEW case from Haribabu's patch,
since I'm not convinced that case is OK, and hacked on the
documentation somewhat.

Discussion: http://postgr.es/m/CAJrrPGdo5bak6qnPWe8Kpi8g_jfQEs-G4SYmG9y+OFaw2-dPvA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e9baa5e9fa147e00a2466ab2c40eb99c8a700824

Modified Files
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doc/src/sgml/parallel.sgml | 16 ++----
src/backend/access/heap/heapam.c | 16 +++---
src/backend/commands/createas.c | 4 +-
src/backend/commands/explain.c | 4 +-
src/backend/executor/execMain.c | 6 +--
src/backend/optimizer/plan/planner.c | 10 ++++
src/test/regress/expected/write_parallel.out | 79 ++++++++++++++++++++++++++++
src/test/regress/parallel_schedule | 1 +
src/test/regress/serial_schedule | 1 +
src/test/regress/sql/write_parallel.sql | 42 +++++++++++++++
10 files changed, 151 insertions(+), 28 deletions(-)