pgsql: Make PlaceHolderInfo lookup O(1).

Make PlaceHolderInfo lookup O(1).

Up to now we've just searched the placeholder_list when we want to
find the PlaceHolderInfo with a given ID.  While there's no evidence
of that being a problem in the field, an upcoming patch will add
find_placeholder_info() calls in build_joinrel_tlist(), which seems
likely to make it more of an issue: a joinrel emitting lots of
PlaceHolderVars would incur O(N^2) cost, and we might be building
a lot of joinrels in complex queries.  Hence, add an array that
can be indexed directly by phid to make the lookups constant-time.

Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6569ca43973b754e8213072c8ddcae9e7baf2aaa

Modified Files
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src/backend/optimizer/plan/analyzejoins.c |  3 +++
src/backend/optimizer/plan/planmain.c     |  2 ++
src/backend/optimizer/util/placeholder.c  | 43 +++++++++++++++++++++++++++----
src/backend/optimizer/util/var.c          | 11 ++------
src/include/nodes/pathnodes.h             |  5 ++++
5 files changed, 50 insertions(+), 14 deletions(-)