pgsql: Improve nbtree unsatisfiable RowCompare detection.

Improve nbtree unsatisfiable RowCompare detection.

Move nbtree's detection of RowCompare quals that are unsatisfiable due
to having a NULL in their first row element: rather than detecting these
cases at the point where _bt_first builds its insertion scan key, do so
earlier, during preprocessing proper. This brings the RowCompare case
in line every other case involving an unsatisfiable-due-to-NULL qual.

nbtree now consistently detects such unsatisfiable quals -- even when
they happen to involve a key that isn't examined by _bt_first at all.
Affected cases thereby avoid useless full index scans that cannot
possibly return any matching rows.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-By: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAH2-WzmySVXst2hFrOATC-zw1Byg1XC-jYUS314=mzuqsNwk+Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ec986020decff322723cf7b3a2696803d082ad17

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 26 +++---
src/backend/access/nbtree/nbtutils.c | 14 +++-
src/test/regress/expected/btree_index.out | 127 +++++++++++++++++++++++++++++
src/test/regress/expected/create_index.out | 13 +++
src/test/regress/sql/btree_index.sql | 79 ++++++++++++++++++
src/test/regress/sql/create_index.sql | 5 ++
6 files changed, 248 insertions(+), 16 deletions(-)