pgsql: Fix calculation in brin_minmax_multi_distance_date

Fix calculation in brin_minmax_multi_distance_date

When calculating the distance between date values, make sure to subtract
them in the right order, i.e. (larger - smaller).

The distance is used to determine which values to merge, and is expected
to be a positive value. The code unfortunately did the subtraction in
the opposite order, i.e. (smaller - larger), thus producing negative
values and merging values the most distant values first.

The resulting index is correct (i.e. produces correct results), but may
be significantly less efficient. This affects all minmax-multi indexes
on date columns.

Backpatch to 14, where minmax-multi indexes were introduced.

Reported-by: Ashutosh Bapat
Reviewed-by: Ashutosh Bapat, Dean Rasheed
Backpatch-through: 14
Discussion: https://postgr.es/m/eef0ea8c-4aaa-8d0d-027f-58b1f35dd170@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/394d51731495794655f95b5580adb757d7a9bbe2

Modified Files
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src/backend/access/brin/brin_minmax_multi.c |  7 ++++++-
src/test/regress/expected/brin_multi.out    | 20 ++++++++++++++++++++
src/test/regress/sql/brin_multi.sql         | 18 ++++++++++++++++++
3 files changed, 44 insertions(+), 1 deletion(-)