pgsql: Delay build of Memoize hash table until executor run

Delay build of Memoize hash table until executor run

Previously this hash table was built during executor startup.  This
could cause long delays in EXPLAIN (without ANALYZE) when the planner
opts to use a large Memoize hash table.

No backpatch for now due to lack of complaints.

Author: David Rowley
Discussion: https://postgr.es/m/CAApHDvoJktJ5XL=Kjh2a2TFr64R-7eQZV-+jcJrUwoES2GLiWg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/57f59396bb51953bb7b957780c7f1b7f67602125

Modified Files
--------------
src/backend/executor/nodeMemoize.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)