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Optimize nbtree backward scan boundary cases.

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Teach _bt_binsrch (and related helper routines like _bt_search and
_bt_compare) about the initial positioning requirements of backward
scans.  Routines like _bt_binsrch already know all about "nextkey"
searches, so it seems natural to teach them about "goback"/backward
searches, too.  These concepts are closely related, and are much easier
to understand when discussed together.

Now that certain implementation details are hidden from _bt_first, it's
straightforward to add a new optimization: backward scans using the <
strategy now avoid extra leaf page accesses in certain "boundary cases".
Consider the following example, which uses the tenk1 table (and its
tenk1_hundred index) from the standard regression tests:

SELECT * FROM tenk1 WHERE hundred < 12 ORDER BY hundred DESC LIMIT 1;

Before this commit, nbtree would scan two leaf pages, even though it was
only really necessary to scan one leaf page.  We'll now descend straight
to the leaf page containing a (12, -inf) high key instead.  The scan
will locate matching non-pivot tuples with "hundred" values starting
from the value 11.  The scan won't waste a page access on the right
sibling leaf page, which cannot possibly contain any matching tuples.

You can think of the optimization added by this commit as disabling an
optimization (the _bt_compare "!pivotsearch" behavior that was added to
Postgres 12 in commit dd299df8) for a small subset of cases where it was
always counterproductive.

Equivalently, you can think of the new optimization as extending the
"pivotsearch" behavior that page deletion by VACUUM has long required
(since the aforementioned Postgres 12 commit went in) to other, similar
cases.  Obviously, this isn't strictly necessary for these new cases
(unlike VACUUM, _bt_first is prepared to move the scan to the left once
on the leaf level), but the underlying principle is the same.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wz=XPzM8HzaLPq278Vms420mVSHfgs9wi5tjFKHcapZCEw@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2023-12-08 11:05:17 -08:00
parent b437571714
commit c9c0589fda
7 changed files with 210 additions and 149 deletions
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12
src/backend/access/nbtree/nbtutils.c
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@ -62,14 +62,6 @@ static int _bt_keep_natts(Relation rel, IndexTuple lastleft,
* Build an insertion scan key that contains comparison data from itup
* as well as comparator routines appropriate to the key datatypes.
*
* When itup is a non-pivot tuple, the returned insertion scan key is
* suitable for finding a place for it to go on the leaf level. Pivot
* tuples can be used to re-find leaf page with matching high key, but
* then caller needs to set scan key's pivotsearch field to true. This
* allows caller to search for a leaf page with a matching high key,
* which is usually to the left of the first leaf page a non-pivot match
* might appear on.
*
* The result is intended for use with _bt_compare() and _bt_truncate().
* Callers that don't need to fill out the insertion scankey arguments
* (e.g. they use an ad-hoc comparison routine, or only need a scankey
@ -120,8 +112,8 @@ _bt_mkscankey(Relation rel, IndexTuple itup)
key->allequalimage = false;
}
key->anynullkeys = false; /* initial assumption */
key->nextkey = false;
key->pivotsearch = false;
key->nextkey = false; /* usual case, required by btinsert */
key->backward = false; /* usual case, required by btinsert */
key->keysz = Min(indnkeyatts, tupnatts);
key->scantid = key->heapkeyspace && itup ?
BTreeTupleGetHeapTID(itup) : NULL;