pgsql: Consider materializing the cheapest inner path in parallel nestl

Consider materializing the cheapest inner path in parallel nestloop

When generating non-parallel nestloop paths for each available outer
path, we always consider materializing the cheapest inner path if
feasible.  Similarly, in this patch, we also consider materializing
the cheapest inner path when building partial nestloop paths.  This
approach potentially reduces the need to rescan the inner side of a
partial nestloop path for each outer tuple.

Author: Tender Wang
Reviewed-by: Richard Guo, Robert Haas, David Rowley, Alena Rybakina
Reviewed-by: Tomasz Rybak, Paul Jungwirth, Yuki Fujii
Discussion: https://postgr.es/m/CAHewXNkPmtEXNfVQMou_7NqQmFABca9f4etjBtdbbm0ZKDmWvw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/22d946b0f86f9c13f869bb8b75444a77a20134d8

Modified Files
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src/backend/optimizer/path/joinpath.c         | 25 ++++++++++++++++++++++
src/test/regress/expected/select_parallel.out | 30 +++++++++++++++++++++++++++
src/test/regress/sql/select_parallel.sql      | 10 +++++++++
3 files changed, 65 insertions(+)