pgsql: Optimize sort and deduplication in ginExtractEntries()

Optimize sort and deduplication in ginExtractEntries()

Remove NULLs from the array first, and use qsort to deduplicate only
the non-NULL items. This simplifies the comparison function. Also
replace qsort_arg() with a templated version so that the comparison
function can be inlined. These changes make ginExtractEntries() a
little faster especially for simple datatypes like integers.

Author: David Geier <geidav(dot)pg(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/6d16b6bd-a1ff-4469-aefb-a1c8274e561a@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6f5ad00ab763f9e029ec591f7f650bd09c1e933f

Modified Files
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src/backend/access/gin/ginutil.c | 151 +++++++++++++++++----------------------
src/include/access/gin_private.h | 2 +-
2 files changed, 68 insertions(+), 85 deletions(-)