c9c0589fda
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
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.