pgsql: Fix tuple counting in SP-GiST index build.

Fix tuple counting in SP-GiST index build.

Count the number of tuples in the index honestly, instead of assuming
that it's the same as the number of tuples in the heap.  (It might be
different if the index is partial.)

Back-patch to all supported versions.

Tomas Vondra

Discussion: https://postgr.es/m/3b3d8eac-c709-0d25-088e-b98339a1b28a@2ndquadrant.com

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7f6f8ccd976da21c0117bddfee8bd86226bd2559

Modified Files
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src/backend/access/spgist/spginsert.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)