pgsql: Track the number of presorted outer pathkeys in MergePath

Track the number of presorted outer pathkeys in MergePath

When creating an explicit Sort node for the outer path of a mergejoin,
we need to determine the number of presorted keys of the outer path to
decide whether explicit incremental sort can be applied.  Currently,
this is done by repeatedly calling pathkeys_count_contained_in.

This patch caches the number of presorted outer pathkeys in MergePath,
allowing us to save several calls to pathkeys_count_contained_in.  It
can be considered a complement to the changes in commit 828e94c9d.

Reported-by: David Rowley <dgrowleyml@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAApHDvqvBireB_w6x8BN5txdvBEHxVgZBt=rUnpf5ww5P_E_ww@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c06e909c26f070dee78f73c35565d6f4a4ffdcda

Modified Files
--------------
src/backend/foreign/foreign.c           |  5 +--
src/backend/optimizer/path/costsize.c   | 59 +++++++++++++++----------------
src/backend/optimizer/path/joinpath.c   | 28 ++++++++++++---
src/backend/optimizer/plan/createplan.c | 61 +++++++++++++++++----------------
src/backend/optimizer/util/pathnode.c   |  5 ++-
src/include/nodes/pathnodes.h           |  8 +++++
src/include/optimizer/cost.h            |  1 +
src/include/optimizer/pathnode.h        |  3 +-
8 files changed, 104 insertions(+), 66 deletions(-)