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David Rowley 99c754129d Fix performance regression in tuplesort specializations 
697492434 added 3 new qsort specialization functions aimed to improve the
performance of sorting many of the common pass-by-value data types when
they're the leading or only sort key.

Unfortunately, that has caused a performance regression when sorting
datasets where many of the values being compared were equal.  What was
happening here was that we were falling back to the standard sort
comparison function to handle tiebreaks.  When the two given Datums
compared equally we would incur both the overhead of an indirect function
call to the standard comparer to perform the tiebreak and also the
standard comparer function would go and compare the leading key needlessly
all over again.

Here improve the situation in the 3 new comparison functions.  We now
return 0 directly when the two Datums compare equally and we're performing
a 1-key sort.

Here we don't do anything to help the multi-key sort case where the
leading key uses one of the sort specializations functions.  On testing
this case, even when the leading key's values are all equal, there
appeared to be no performance regression.  Let's leave it up to future
work to optimize that case so that the tiebreak function no longer
re-compares the leading key over again.

Another possible fix for this would have been to add 3 additional sort
specialization functions to handle single-key sorts for these
pass-by-value types.  The reason we didn't do that here is that we may
deem some other sort specialization to be more useful than single-key
sorts.  It may be impractical to have sort specialization functions for
every single combination of what may be useful and it was already decided
that further analysis into which ones are the most useful would be delayed
until the v16 cycle.  Let's not let this regression force our hand into
trying to make that decision for v15.

Author: David Rowley
Reviewed-by: John Naylor
Discussion: https://postgr.es/m/CA+hUKGJRbzaAOUtBUcjF5hLtaSHnJUqXmtiaLEoi53zeWSizeA@mail.gmail.com
 		2022-04-22 16:02:15 +12:00
config Update config.guess and config.sub 2022-04-07 07:45:25 +02:00
contrib postgres_fdw: Disable batch insert when BEFORE ROW INSERT triggers exist. 2022-04-21 15:30:00 +09:00
doc Rethink method for assigning OIDs to the template0 and postgres DBs. 2022-04-21 16:23:15 -04:00
src Fix performance regression in tuplesort specializations 2022-04-22 16:02:15 +12:00
.cirrus.yml Make upgradecheck a no-op in MSVC's vcregress.pl 2022-04-02 12:06:11 +09:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
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.git-blame-ignore-revs Add a few recent and not so recent revs to git-blame-ignore-revs. 2022-03-19 12:37:28 -07:00
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.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
COPYRIGHT Update copyright for 2022 2022-01-07 19:04:57 -05:00
GNUmakefile.in add missing tag from commit b8c4261e5e 2021-07-01 15:47:46 -04:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
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README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00
aclocal.m4 Probe $PROVE not $PERL while checking for modules needed by TAP tests. 2021-11-22 12:54:52 -05:00
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README 

PostgreSQL Database Management System
=====================================

This directory contains the source code distribution of the PostgreSQL
database management system.

PostgreSQL is an advanced object-relational database management system
that supports an extended subset of the SQL standard, including
transactions, foreign keys, subqueries, triggers, user-defined types
and functions.  This distribution also contains C language bindings.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download/

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
https://www.postgresql.org/download/.  For more information look at our
web site located at https://www.postgresql.org/.