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 <p...@bowt.ie> Reviewed-By: Matthias van de Meent <boekewurm+postg...@gmail.com> Discussion: https://postgr.es/m/CAH2-Wz=FJ78K3WsF3iWNxWnUCY9f=Jdg3QPxaXE=uyubmur...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/b3f1a13f22f9e28842ee5fbd08b7ec805e27aaac Modified Files -------------- src/backend/access/nbtree/nbtpreprocesskeys.c | 180 ++++++++++++++++++++++++++ src/test/regress/expected/create_index.out | 21 +++ src/test/regress/sql/create_index.sql | 10 ++ 3 files changed, 211 insertions(+)
