pgsql: Preparatory refactoring for parallel merge join support.

Preparatory refactoring for parallel merge join support.

Extract the logic used by hash_inner_and_outer into a separate
function, get_cheapest_parallel_safe_total_inner, so that it can
also be used to plan parallel merge joins.

Also, add a require_parallel_safe argument to the existing function
get_cheapest_path_for_pathkeys, because parallel merge join needs
to find the cheapest path for a given set of pathkeys that is
parallel-safe, not just the cheapest one overall.

Patch by me, reviewed by Dilip Kumar.

Discussion: http://postgr.es/m/CA+TgmoYOv+dFK0MWW6366dFj_xTnohQfoBDrHyB7d1oZhrgPjA@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a71f10189dc10a2fe422158a2c9409e0f77c6b9e

Modified Files
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src/backend/optimizer/path/allpaths.c | 9 ++++++---
src/backend/optimizer/path/joinpath.c | 23 ++++++-----------------
src/backend/optimizer/path/pathkeys.c | 29 ++++++++++++++++++++++++++++-
src/include/optimizer/paths.h | 4 +++-
4 files changed, 43 insertions(+), 22 deletions(-)