pgsql: Fix incorrect return value in brin_minmax_multi_distance_numeric

Fix incorrect return value in brin_minmax_multi_distance_numeric().

The result of "DirectFunctionCall1(numeric_float8, d)" is already in
Datum form, but the code was incorrectly applying PG_RETURN_FLOAT8()
to it. On machines where float8 is pass-by-reference, this would
result in complete garbage, since an unpredictable pointer value
would be treated as an integer and then converted to float. It's not
entirely clear how much of a problem would ensue on 64-bit hardware,
but certainly interpreting a float8 bitpattern as uint64 and then
converting that to float isn't the intended behavior.

As luck would have it, even the complete-garbage case doesn't break
BRIN indexes, since the results are only used to make choices about
how to merge values into ranges: at worst, we'd make poor choices
resulting in an inefficient index. Doubtless that explains the lack
of field complaints. However, users with BRIN indexes that use the
numeric_minmax_multi_ops opclass may wish to reindex in hopes of
making their indexes more efficient.

Author: Peter Eisentraut <peter(at)eisentraut(dot)org>
Co-authored-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9b681e239726d254b18cf88ad94174e086bf1950

Modified Files
--------------
src/backend/access/brin/brin_minmax_multi.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)