pgsql: Allow simplehash to use already-calculated hash values.

Allow simplehash to use already-calculated hash values.

Add _lookup_hash and _insert_hash functions for callers that have
already calculated the hash value of the key.

The immediate use case is for hash algorithms that write to disk in
partitions. The hash value can be calculated once, used to perform a
lookup, used to select the partition, then written to the partition
along with the tuple. When the tuple is read back, the hash value does
not need to be recalculated.

Author: Jeff Davis
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/48abe675e1330f0c264ab2fe0d4ff23eb244f9ef.camel%40j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6ae4e8eae78e0781633f7b40a1b5cc189bc40923

Modified Files
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src/include/lib/simplehash.h | 74 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 65 insertions(+), 9 deletions(-)