pgsql: Speedup Hash Joins with dedicated functions for ExprState hashin

Speedup Hash Joins with dedicated functions for ExprState hashing

Hashing of a single Var is a very common operation for ExprState to
perform. Here we add dedicated ExecJust* functions which helps speed up
Hash Joins by removing the interpretation overhead in ExecInterpExpr().

This change currently only affects Hash Joins on a single column. Hash
Joins with multiple join keys or an expression still run through
ExecInterpExpr().

Some testing has shown up to 10% query performance increases on recent AMD
hardware and nearly 7% increase on an Apple M2 for a query performing a
hash join with a large number of lookups on a small hash table.

This change was extracted from a larger patch which adjusts GROUP BY /
hashed subplans / hashed set operations to use ExprState hashing.

Discussion: https://postgr.es/m/CAApHDvr8Zc0ZgzVoCZLdHGOFNhiJeQ6vrUcS9V7N23zMWQb-eA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/50416cc4843a85fcb53507e21577cce16c75c65f

Modified Files
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src/backend/executor/execExprInterp.c | 206 +++++++++++++++++++++++++++++++++-
1 file changed, 205 insertions(+), 1 deletion(-)