Avoid extra index searches through preprocessing.

Avoid extra index searches through preprocessing.

Transform low_compare and high_compare nbtree skip array inequalities
(with opclasses that offer skip support) in such a way as to allow
_bt_first to consistently apply later keys when it descends the tree.
This can lower the number of index searches for multi-column scans that
use a ">" key on one of the index's prefix columns (or use a "<" key,
when scanning backwards) when it precedes some later lower-order key.

For example, an index qual "WHERE a > 5 AND b = 2" will now be converted
to "WHERE a >= 6 AND b = 2" by a new preprocessing step that takes place
after low_compare and high_compare have been finalized.  That way, the
initial call to _bt_first can use "WHERE a >= 6 AND b = 2" to find an
initial position, rather than just using "WHERE a > 5" -- "b = 2" can be
applied during every _bt_first call.  There's a decent chance that this
will allow such a scan to avoid the extra search that might otherwise be
needed to determine the lowest "a" value still satisfying "WHERE a > 5".

The transformation process can only lower the total number of index
pages read when the use of a more restrictive set of initial positioning
keys in _bt_first actually allows the scan to land on some later leaf
page directly, relative to the unoptimized case (or on an earlier leaf
page directly, when scanning backwards).  But the savings can really add
up in cases where an affected skip array comes after some other array.
For example, a scan indexqual "WHERE x IN (1, 2, 3) AND y > 5 AND z = 2"
can save as many as 3 _bt_first calls by applying the new transformation
to its "y" array (up to 1 extra search can be avoided per "x" element).

Follow-up to commit 92fe23d9, which added nbtree skip scan.

Author: Peter Geoghegan <[email protected]>
Reviewed-By: Matthias van de Meent <[email protected]>
Discussion: https://postgr.es/m/CAH2-Wz=FJ78K3WsF3iWNxWnUCY9f=Jdg3QPxaXE=uYUbmuRz5Q@mail.gmail.com