pgsql: Improvements and fixes for e0b1ee17dc

Improvements and fixes for e0b1ee17dc

e0b1ee17dc introduced optimization for matching B-tree scan keys required for
the directional scan.  However, it incorrectly assumed that all keys required
for opposite direction scan are satisfied by _bt_first().  It has been
illustrated that with multiple scan keys over the same column, a lesser one
(according to the scan direction) could win leaving the other one unsatisfied.

Instead of relying on _bt_first() this commit introduces code that memorizes
whether there was at least one match on the page.  If that's true we know that
keys required for opposite-direction scan are satisfied as soon as
corresponding values are not NULLs.

Also, this commit simplifies the description for the optimization of keys
required for the current direction scan.  Now the flag used for this is named
continuescanPrechecked and means exactly that *continuescan flag is known
to be true for the last item on the page.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzn0LeLcb1PdBnK0xisz8NpHkxRrMr3NWJ%2BKOK-WZ%2BQtTQ%40mail.gmail.com
Reviewed-by: Pavel Borisov

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7e6fb5da41d8ee1bddcd5058b7204018ef68d193

Modified Files
--------------
src/backend/access/nbtree/nbtsearch.c | 42 ++++++++++++++++++++---------------
src/backend/access/nbtree/nbtutils.c  | 40 +++++++++++++++++++++------------
src/include/access/nbtree.h           |  2 +-
3 files changed, 51 insertions(+), 33 deletions(-)