pgsql: De-dupicate Memoize cache keys

De-dupicate Memoize cache keys

It was possible when determining the cache keys for a Memoize path that
if the same expr appeared twice in the parameterized path's ppi_clauses
and/or in the Nested Loop's inner relation's lateral_vars.  If this
happened the Memoize node's cache keys would contain duplicates.  This
isn't a problem for correctness, all it means is that the cache lookups
will be suboptimal due to having redundant work to do on every hash table
lookup and insert.

Here we adjust paraminfo_get_equal_hashops() to look for duplicates and
ignore them when we find them.

Author: David Rowley
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/422277.1706207562%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bc397e5cdb31c399392d693cbc5909341d21235a

Modified Files
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src/backend/optimizer/path/joinpath.c | 24 ++++++++++++++++++++----
src/test/regress/expected/memoize.out | 2 +-
2 files changed, 21 insertions(+), 5 deletions(-)