pgsql: Reduce memory used by partitionwise joins

Reduce memory used by partitionwise joins

Specifically, this commit reduces the memory consumed by the
SpecialJoinInfos that are allocated for child joins in
try_partitionwise_join() by freeing them at the end of creating paths
for each child join.

A SpecialJoinInfo allocated for a given child join is a copy of the
parent join's SpecialJoinInfo, which contains the translated copies
of the various Relids bitmapsets and semi_rhs_exprs, which is a List
of Nodes. The newly added freeing step frees the struct itself and
the various bitmapsets, but not semi_rhs_exprs, because there's no
handy function to free the memory of Node trees.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Amit Langote <amitlangote09(at)gmail(dot)com>
Reviewed-by: Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
Reviewed-by: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/CAExHW5tHqEf3ASVqvFFcghYGPfpy7o3xnvhHwBGbJFMRH8KjNw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5278d0a2e870c61f9374a7796b90e6f9f6a73638

Modified Files
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src/backend/optimizer/path/joinrels.c | 36 +++++++++++++++++++++++++++++++++++
src/backend/optimizer/util/pathnode.c | 5 +++--
src/include/nodes/pathnodes.h | 3 +++
3 files changed, 42 insertions(+), 2 deletions(-)