pgsql: Initialize ExprStates once in run-time partition pruning

Initialize ExprStates once in run-time partition pruning

Instead of doing ExecInitExpr every time a Param needs to be evaluated
in run-time partition pruning, do it once during run-time pruning
set-up and cache the exprstate in PartitionPruneContext, saving a lot of
work.

Author: David Rowley
Reviewed-by: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/CAKJS1f8-x+q-90QAPDu_okhQBV4DPEtPz8CJ=m0940GyT4DA4w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1957f8dabf8daa29c78d05f971dd665c9680a754

Modified Files
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src/backend/executor/execPartition.c | 35 +++++++++++++++++++++++++++++++++++
src/backend/partitioning/partprune.c | 27 +++++++++++++++++----------
src/include/partitioning/partprune.h | 9 +++++++++
3 files changed, 61 insertions(+), 10 deletions(-)