From pgsql-performance-owner+M17204@postgresql.org Wed Feb 15 16:28:34 2006
Return-path: <pgsql-performance-owner+M17204@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k1FLSV527014
	for <pgman@candle.pha.pa.us>; Wed, 15 Feb 2006 16:28:31 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 168C967B584;
	Wed, 15 Feb 2006 17:28:29 -0400 (AST)
X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id BB0AB9DCB9E
	for <pgsql-performance-postgresql.org@localhost.postgresql.org>; Wed, 15 Feb 2006 17:27:56 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 22055-07
	for <pgsql-performance-postgresql.org@localhost.postgresql.org>;
	Wed, 15 Feb 2006 17:27:57 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id F385E9DCB98
	for <pgsql-performance@postgresql.org>; Wed, 15 Feb 2006 17:27:53 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k1FLRsqd019780;
	Wed, 15 Feb 2006 16:27:54 -0500 (EST)
To: Gary Doades <gpd@gpdnet.co.uk>
cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Strange Create Index behaviour 
In-Reply-To: <19510.1140036968@sss.pgh.pa.us> 
References: <43F38867.6010701@gpdnet.co.uk> <19510.1140036968@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Wed, 15 Feb 2006 15:56:08 -0500"
Date: Wed, 15 Feb 2006 16:27:54 -0500
Message-ID: <19779.1140038874@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.11 required=5 tests=[AWL=0.110]
X-Spam-Score: 0.11
X-Mailing-List: pgsql-performance
List-Archive: <http://archives.postgresql.org/pgsql-performance>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-performance.postgresql.org>
List-Owner: <mailto:pgsql-performance-owner@postgresql.org>
List-Post: <mailto:pgsql-performance@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-performance>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org
Status: ORr

I wrote:
> Interesting.  I tried your test script and got fairly close times
> for all the cases on two different machines:
> 	old HPUX machine: shortest 5800 msec, longest 7960 msec
> 	new Fedora 4 machine: shortest 461 msec, longest 608 msec

> So what this looks like to me is a corner case that FreeBSD's qsort
> fails to handle well.

I tried forcing PG to use src/port/qsort.c on the Fedora machine,
and lo and behold:
	new Fedora 4 machine: shortest 434 msec, longest 8530 msec

So it sure looks like this script does expose a problem on BSD-derived
qsorts.  Curiously, the case that's much the worst for me is the third
in the script, while the shortest time is the first case, which was slow
for Gary.  So I'd venture that the *BSD code has been tweaked somewhere
along the way, in a manner that moves the problem around without really
fixing it.  (Anyone want to compare the actual FreeBSD source to what
we have?)

This is pretty relevant stuff, because there was a thread recently
advocating that we stop using the platform qsort on all platforms:
http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php

It's really interesting to see a case where port/qsort is radically
worse than other qsorts ... unless we figure that out and fix it,
I think the idea of using port/qsort everywhere has just taken a
major hit.

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

From pgsql-performance-owner+M17212@postgresql.org Wed Feb 15 18:29:07 2006
Return-path: <pgsql-performance-owner+M17212@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k1FNT6509074
	for <pgman@candle.pha.pa.us>; Wed, 15 Feb 2006 18:29:06 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 2BE6267B58B;
	Wed, 15 Feb 2006 19:29:04 -0400 (AST)
X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 7C3D49DC803;
	Wed, 15 Feb 2006 19:28:30 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 47149-10; Wed, 15 Feb 2006 19:28:32 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id C56AD9DC843;
	Wed, 15 Feb 2006 19:28:27 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k1FNSTkm020782;
	Wed, 15 Feb 2006 18:28:29 -0500 (EST)
To: Gary Doades <gpd@gpdnet.co.uk>
cc: pgsql-performance@postgresql.org, pgsql-hackers@postgresql.org
Subject: qsort again (was Re: [PERFORM] Strange Create Index behaviour)
In-Reply-To: <43F39E53.1020009@gpdnet.co.uk> 
References: <43F38867.6010701@gpdnet.co.uk> <19510.1140036968@sss.pgh.pa.us> <19779.1140038874@sss.pgh.pa.us> <43F39E53.1020009@gpdnet.co.uk>
Comments: In-reply-to Gary Doades <gpd@gpdnet.co.uk>
	message dated "Wed, 15 Feb 2006 21:34:11 +0000"
Date: Wed, 15 Feb 2006 18:28:29 -0500
Message-ID: <20781.1140046109@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.11 required=5 tests=[AWL=0.110]
X-Spam-Score: 0.11
X-Mailing-List: pgsql-performance
List-Archive: <http://archives.postgresql.org/pgsql-performance>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-performance.postgresql.org>
List-Owner: <mailto:pgsql-performance-owner@postgresql.org>
List-Post: <mailto:pgsql-performance@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-performance>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org
Status: OR

Gary Doades <gpd@gpdnet.co.uk> writes:
> If I run the script again, it is not always the first case that is slow, 
> it varies from run to run, which is why I repeated it quite a few times 
> for the test.

For some reason I hadn't immediately twigged to the fact that your test
script is just N repetitions of the exact same structure with random data.
So it's not so surprising that you get random variations in behavior
with different test data sets.

I did some experimentation comparing the qsort from Fedora Core 4
(glibc-2.3.5-10.3) with our src/port/qsort.c.  For those who weren't
following the pgsql-performance thread, the test case is just this
repeated a lot of times:

create table atest(i int4, r int4);
insert into atest (i,r) select generate_series(1,100000), 0;
insert into atest (i,r) select generate_series(1,100000), random()*100000;
\timing
create index idx on atest(r);
\timing
drop table atest;

I did this 100 times and sorted the reported runtimes.  (Investigation
with trace_sort = on confirms that the runtime is almost entirely spent
in qsort() called from our performsort --- the Postgres overhead is
about 100msec on this machine.)  Results are below.

It seems clear that our qsort.c is doing a pretty awful job of picking
qsort pivots, while glibc is mostly managing not to make that mistake.
I haven't looked at the glibc code yet to see what they are doing
differently.

I'd say this puts a considerable damper on my enthusiasm for using our
qsort all the time, as was recently debated in this thread:
http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php
We need to fix our qsort.c before pushing ahead with that idea.

			regards, tom lane


100 runtimes for glibc qsort, sorted ascending:

Time: 459.860 ms
Time: 460.209 ms
Time: 460.704 ms
Time: 461.317 ms
Time: 461.538 ms
Time: 461.652 ms
Time: 461.988 ms
Time: 462.573 ms
Time: 462.638 ms
Time: 462.716 ms
Time: 462.917 ms
Time: 463.219 ms
Time: 463.455 ms
Time: 463.650 ms
Time: 463.723 ms
Time: 463.737 ms
Time: 463.750 ms
Time: 463.852 ms
Time: 463.964 ms
Time: 463.988 ms
Time: 464.003 ms
Time: 464.135 ms
Time: 464.372 ms
Time: 464.458 ms
Time: 464.496 ms
Time: 464.551 ms
Time: 464.599 ms
Time: 464.655 ms
Time: 464.656 ms
Time: 464.722 ms
Time: 464.814 ms
Time: 464.827 ms
Time: 464.878 ms
Time: 464.899 ms
Time: 464.905 ms
Time: 464.987 ms
Time: 465.055 ms
Time: 465.138 ms
Time: 465.159 ms
Time: 465.194 ms
Time: 465.310 ms
Time: 465.316 ms
Time: 465.375 ms
Time: 465.450 ms
Time: 465.535 ms
Time: 465.595 ms
Time: 465.680 ms
Time: 465.769 ms
Time: 465.865 ms
Time: 465.892 ms
Time: 465.903 ms
Time: 466.003 ms
Time: 466.154 ms
Time: 466.164 ms
Time: 466.203 ms
Time: 466.305 ms
Time: 466.344 ms
Time: 466.364 ms
Time: 466.388 ms
Time: 466.502 ms
Time: 466.593 ms
Time: 466.725 ms
Time: 466.794 ms
Time: 466.798 ms
Time: 466.904 ms
Time: 466.971 ms
Time: 466.997 ms
Time: 467.122 ms
Time: 467.146 ms
Time: 467.221 ms
Time: 467.224 ms
Time: 467.244 ms
Time: 467.277 ms
Time: 467.587 ms
Time: 468.142 ms
Time: 468.207 ms
Time: 468.237 ms
Time: 468.471 ms
Time: 468.663 ms
Time: 468.700 ms
Time: 469.235 ms
Time: 469.840 ms
Time: 470.472 ms
Time: 471.140 ms
Time: 472.811 ms
Time: 472.959 ms
Time: 474.858 ms
Time: 477.210 ms
Time: 479.571 ms
Time: 479.671 ms
Time: 482.797 ms
Time: 488.852 ms
Time: 514.639 ms
Time: 529.287 ms
Time: 612.185 ms
Time: 660.748 ms
Time: 742.227 ms
Time: 866.814 ms
Time: 1234.848 ms
Time: 1267.398 ms


100 runtimes for port/qsort.c, sorted ascending:

Time: 418.905 ms
Time: 420.611 ms
Time: 420.764 ms
Time: 420.904 ms
Time: 421.706 ms
Time: 422.466 ms
Time: 422.627 ms
Time: 423.189 ms
Time: 423.302 ms
Time: 425.096 ms
Time: 425.731 ms
Time: 425.851 ms
Time: 427.253 ms
Time: 430.113 ms
Time: 432.756 ms
Time: 432.963 ms
Time: 440.502 ms
Time: 440.640 ms
Time: 450.452 ms
Time: 458.143 ms
Time: 459.212 ms
Time: 467.706 ms
Time: 468.006 ms
Time: 468.574 ms
Time: 470.003 ms
Time: 472.313 ms
Time: 483.622 ms
Time: 492.395 ms
Time: 509.564 ms
Time: 531.037 ms
Time: 533.366 ms
Time: 535.610 ms
Time: 575.523 ms
Time: 582.688 ms
Time: 593.545 ms
Time: 647.364 ms
Time: 660.612 ms
Time: 677.312 ms
Time: 680.288 ms
Time: 697.626 ms
Time: 833.066 ms
Time: 834.511 ms
Time: 851.819 ms
Time: 920.443 ms
Time: 926.731 ms
Time: 954.289 ms
Time: 1045.214 ms
Time: 1059.200 ms
Time: 1062.328 ms
Time: 1136.018 ms
Time: 1260.091 ms
Time: 1276.883 ms
Time: 1319.351 ms
Time: 1438.854 ms
Time: 1475.457 ms
Time: 1538.211 ms
Time: 1549.004 ms
Time: 1744.642 ms
Time: 1771.258 ms
Time: 1959.530 ms
Time: 2300.140 ms
Time: 2589.641 ms
Time: 2612.780 ms
Time: 3100.024 ms
Time: 3284.125 ms
Time: 3379.792 ms
Time: 3750.278 ms
Time: 4302.278 ms
Time: 4780.624 ms
Time: 5000.056 ms
Time: 5092.604 ms
Time: 5168.722 ms
Time: 5292.941 ms
Time: 5895.964 ms
Time: 7003.164 ms
Time: 7099.449 ms
Time: 7115.083 ms
Time: 7384.940 ms
Time: 8214.010 ms
Time: 8700.771 ms
Time: 9331.225 ms
Time: 10503.360 ms
Time: 12496.026 ms
Time: 12982.474 ms
Time: 15192.390 ms
Time: 15392.161 ms
Time: 15958.295 ms
Time: 18375.693 ms
Time: 18617.706 ms
Time: 18927.515 ms
Time: 19898.018 ms
Time: 20865.979 ms
Time: 21000.907 ms
Time: 21297.585 ms
Time: 21714.518 ms
Time: 25423.235 ms
Time: 27543.052 ms
Time: 28314.182 ms
Time: 29400.278 ms
Time: 34142.534 ms

---------------------------(end of broadcast)---------------------------
TIP 1: if posting/reading through Usenet, please send an appropriate
       subscribe-nomail command to majordomo@postgresql.org so that your
       message can get through to the mailing list cleanly

From pgsql-hackers-owner+M79733@postgresql.org Wed Feb 15 20:22:07 2006
Return-path: <pgsql-hackers-owner+M79733@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k1G1M6529533
	for <pgman@candle.pha.pa.us>; Wed, 15 Feb 2006 20:22:06 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id E5C5467B58F;
	Wed, 15 Feb 2006 21:22:03 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 3DAA69DCACE;
	Wed, 15 Feb 2006 21:21:34 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 76351-01; Wed, 15 Feb 2006 21:21:36 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id 2FBB59DCA3F;
	Wed, 15 Feb 2006 21:21:31 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k1G1LXXi021616;
	Wed, 15 Feb 2006 20:21:33 -0500 (EST)
To: Ron <rjpeace@earthlink.net>
cc: pgsql-performance@postgresql.org, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) 
In-Reply-To: <7.0.1.0.2.20060215194635.03b55da0@earthlink.net> 
References: <43F38867.6010701@gpdnet.co.uk> <19510.1140036968@sss.pgh.pa.us> <19779.1140038874@sss.pgh.pa.us> <43F39E53.1020009@gpdnet.co.uk> <20781.1140046109@sss.pgh.pa.us> <7.0.1.0.2.20060215194635.03b55da0@earthlink.net>
Comments: In-reply-to Ron <rjpeace@earthlink.net>
	message dated "Wed, 15 Feb 2006 19:57:51 -0500"
Date: Wed, 15 Feb 2006 20:21:33 -0500
Message-ID: <21615.1140052893@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.11 required=5 tests=[AWL=0.110]
X-Spam-Score: 0.11
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

Ron <rjpeace@earthlink.net> writes:
> How are we choosing our pivots?

See qsort.c: it looks like median of nine equally spaced inputs (ie,
the 1/8th points of the initial input array, plus the end points),
implemented as two rounds of median-of-three choices.  With half of the
data inputs zero, it's not too improbable for two out of the three
samples to be zeroes in which case I think the med3 result will be zero
--- so choosing a pivot of zero is much more probable than one would
like, and doing so in many levels of recursion causes the problem.

I think.  I'm not too sure if the code isn't just being sloppy about the
case where many data values are equal to the pivot --- there's a special
case there to switch to insertion sort, and maybe that's getting invoked
too soon.  It'd be useful to get a line-level profile of the behavior of
this code in the slow cases...

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?

               http://www.postgresql.org/docs/faq

From pgsql-performance-owner+M17282@postgresql.org Fri Feb 17 23:11:11 2006
Return-path: <pgsql-performance-owner+M17282@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k1I4BA515503
	for <pgman@candle.pha.pa.us>; Fri, 17 Feb 2006 23:11:10 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 2825F67B5F5;
	Sat, 18 Feb 2006 00:11:07 -0400 (AST)
X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 7BB8A9DCC4F;
	Wed, 15 Feb 2006 21:37:57 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 79365-02; Wed, 15 Feb 2006 21:38:00 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from postal.corporate.connx.com (postal.corporate.connx.com [65.212.159.187])
	by postgresql.org (Postfix) with ESMTP id 33BEA9DCACE;
	Wed, 15 Feb 2006 21:37:54 -0400 (AST)
X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) 
Date: Wed, 15 Feb 2006 17:37:58 -0800
Message-ID: <D425483C2C5C9F49B5B7A41F8944154757D54C@postal.corporate.connx.com>
Thread-Topic: [HACKERS] qsort again (was Re: [PERFORM] Strange Create Index behaviour) 
Thread-Index: AcYyl2fPgxfNXHIRRyOEN4ZGeHtA3wAAEaNQ
From: "Dann Corbit" <DCorbit@connx.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>, "Ron" <rjpeace@earthlink.net>
cc: <pgsql-performance@postgresql.org>, <pgsql-hackers@postgresql.org>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.075 required=5 tests=[AWL=0.075]
X-Spam-Score: 0.075
X-Mailing-List: pgsql-performance
List-Archive: <http://archives.postgresql.org/pgsql-performance>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-performance.postgresql.org>
List-Owner: <mailto:pgsql-performance-owner@postgresql.org>
List-Post: <mailto:pgsql-performance@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-performance>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id k1I4BA515503
Status: ORr



> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-
> owner@postgresql.org] On Behalf Of Tom Lane
> Sent: Wednesday, February 15, 2006 5:22 PM
> To: Ron
> Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create
Index
> behaviour)
> 
> Ron <rjpeace@earthlink.net> writes:
> > How are we choosing our pivots?
> 
> See qsort.c: it looks like median of nine equally spaced inputs (ie,
> the 1/8th points of the initial input array, plus the end points),
> implemented as two rounds of median-of-three choices.  With half of
the
> data inputs zero, it's not too improbable for two out of the three
> samples to be zeroes in which case I think the med3 result will be
zero
> --- so choosing a pivot of zero is much more probable than one would
> like, and doing so in many levels of recursion causes the problem.

Adding some randomness to the selection of the pivot is a known
technique to fix the oddball partitions problem.  However, Bentley and
Sedgewick proved that every quick sort algorithm has some input set that
makes it go quadratic (hence the recent popularity of introspective
sort, which switches to heapsort if quadratic behavior is detected.  The
C++ template I submitted was an example of introspective sort, but
PostgreSQL does not use C++ so it was not helpful).

> I think.  I'm not too sure if the code isn't just being sloppy about
the
> case where many data values are equal to the pivot --- there's a
special
> case there to switch to insertion sort, and maybe that's getting
invoked
> too soon.  

Here are some cases known to make qsort go quadratic:
1. Data already sorted
2. Data reverse sorted
3. Data organ-pipe sorted or ramp
4. Almost all data of the same value

There are probably other cases.  Randomizing the pivot helps some, as
does check for in-order or reverse order partitions.

Imagine if 1/3 of the partitions fall into a category that causes
quadratic behavior (have one of the above formats and have more than
CUTOFF elements in them).

It is doubtful that the switch to insertion sort is causing any sort of
problems.  It is only going to be invoked on tiny sets, for which it has
a fixed cost that is probably less that qsort() function calls on sets
of the same size.

>It'd be useful to get a line-level profile of the behavior of
> this code in the slow cases...

I guess that my in-order or presorted tests [which often arise when
there are very few distinct values] may solve the bad partition
problems.  Don't forget that the algorithm is called recursively.

> 			regards, tom lane
> 
> ---------------------------(end of
broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
> 
>                http://www.postgresql.org/docs/faq

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

From kleptog@svana.org Mon Dec 19 06:37:51 2005
Return-path: <kleptog@svana.org>
Received: from svana.org (mail@svana.org [203.20.62.76])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id jBJBboe20936
	for <pgman@candle.pha.pa.us>; Mon, 19 Dec 2005 06:37:51 -0500 (EST)
Received: from kleptog by svana.org with local (Exim 3.35 #1 (Debian))
	id 1EoJKc-00045V-00; Mon, 19 Dec 2005 22:37:30 +1100
Date: Mon, 19 Dec 2005 12:37:30 +0100
From: Martijn van Oosterhout <kleptog@svana.org>
To: Dann Corbit <DCorbit@connx.com>
cc: Tom Lane <tgl@sss.pgh.pa.us>, Qingqing Zhou <zhouqq@cs.toronto.edu>,
   Bruce Momjian <pgman@candle.pha.pa.us>,
   Luke Lonergan <llonergan@greenplum.com>, Neil Conway <neilc@samurai.com>,
   pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Re: Which qsort is used
Message-ID: <20051219113724.GD12251@svana.org>
Reply-To: Martijn van Oosterhout <kleptog@svana.org>
References: <D425483C2C5C9F49B5B7A41F8944154757D38D@postal.corporate.connx.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
	protocol="application/pgp-signature"; boundary="5gxpn/Q6ypwruk0T"
Content-Disposition: inline
In-Reply-To: <D425483C2C5C9F49B5B7A41F8944154757D38D@postal.corporate.connx.com>
User-Agent: Mutt/1.3.28i
X-PGP-Key-ID: Length=1024; ID=0x0DC67BE6
X-PGP-Key-Fingerprint: 295F A899 A81A 156D B522  48A7 6394 F08A 0DC6 7BE6
X-PGP-Key-URL: <http://svana.org/kleptog/0DC67BE6.pgp.asc>
Status: OR


--5gxpn/Q6ypwruk0T
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Fri, Dec 16, 2005 at 10:43:58PM -0800, Dann Corbit wrote:
> I am actually quite impressed with the excellence of Bentley's sort out
> of the box.  It's definitely the best library implementation of a sort I
> have seen.

I'm not sure whether we have a conclusion here, but I do have one
question: is there a significant difference in the number of times the
comparison routines are called? Comparisons in PostgreSQL are fairly
expensive given the fmgr overhead and when comparing tuples it's even
worse.

We don't want to accedently pick a routine that saves data shuffling by
adding extra comparisons. The stats at [1] don't say. They try to
factor in CPU cost but they seem to use unrealistically small values. I
would think a number around 50 (or higher) would be more
representative.

[1] http://www.cs.toronto.edu/~zhouqq/postgresql/sort/sort.html

Have a nice day,
--=20
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

--5gxpn/Q6ypwruk0T
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.6 (GNU/Linux)
Comment: For info see http://www.gnupg.org

iD8DBQFDpptzIB7bNG8LQkwRAmC6AJ4qYrIm3SYnBV3BybSmm+Gl4vpEywCfRDxg
bnIK4INRqOVFNBAKR/gDPcM=
=92qA
-----END PGP SIGNATURE-----

--5gxpn/Q6ypwruk0T--

From mkoi-pg@aon.at Wed Dec 21 19:44:03 2005
Return-path: <mkoi-pg@aon.at>
Received: from email.aon.at (warsl404pip5.highway.telekom.at [195.3.96.77])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id jBM0i2e05649
	for <pgman@candle.pha.pa.us>; Wed, 21 Dec 2005 19:44:02 -0500 (EST)
Received: (qmail 12703 invoked from network); 22 Dec 2005 00:43:51 -0000
Received: from m148p015.dipool.highway.telekom.at (HELO Sokrates) ([62.46.8.111])
          (envelope-sender <mkoi-pg@aon.at>)
          by smarthub78.highway.telekom.at (qmail-ldap-1.03) with SMTP
          for <tgl@sss.pgh.pa.us>; 22 Dec 2005 00:43:51 -0000
From: Manfred Koizar <mkoi-pg@aon.at>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "Dann Corbit" <DCorbit@connx.com>, "Qingqing Zhou" <zhouqq@cs.toronto.edu>,
   "Bruce Momjian" <pgman@candle.pha.pa.us>,
   "Luke Lonergan" <llonergan@greenplum.com>,
   "Neil Conway" <neilc@samurai.com>, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Re: Which qsort is used 
Date: Thu, 22 Dec 2005 01:43:34 +0100
Message-ID: <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com>
References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us>
In-Reply-To: <3148.1134795805@sss.pgh.pa.us>
X-Mailer: Forte Agent 3.1/32.783
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Status: OR

On Sat, 17 Dec 2005 00:03:25 -0500, Tom Lane <tgl@sss.pgh.pa.us>
wrote:
>I've still got a problem with these checks; I think they are a net
>waste of cycles on average.  [...]
> and when they fail, those cycles are entirely wasted;
>you have not advanced the state of the sort at all.

How can we make the initial check "adavance the state of the sort"?
One answer might be to exclude the sorted sequence at the start of the
array from the qsort, and merge the two sorted lists as the final
stage of the sort.

Qsorting N elements costs O(N*lnN), so excluding H elements from the
sort reduces the cost by at least O(H*lnN).  The merge step costs O(N)
plus some (<=50%) more memory, unless someone knows a fast in-place
merge.  So depending on the constant factors involved there might be a
usable solution.

I've been playing with some numbers and assuming the constant factors
to be equal for all the O()'s this method starts to pay off at
	  H      for N
	  20       100
	 130      1000
	8000    100000
Servus
 Manfred

From pgsql-hackers-owner+M77795=pgman=candle.pha.pa.us@postgresql.org Thu Dec 22 02:02:28 2005
Return-path: <pgsql-hackers-owner+M77795=pgman=candle.pha.pa.us@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id jBM72Re16910
	for <pgman@candle.pha.pa.us>; Thu, 22 Dec 2005 02:02:28 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id A31E067AAA0
	for <pgman@candle.pha.pa.us>; Thu, 22 Dec 2005 03:02:22 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 2C8EC9DCA92
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 22 Dec 2005 03:01:56 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 26033-04
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 22 Dec 2005 03:01:55 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from svana.org (svana.org [203.20.62.76])
	by postgresql.org (Postfix) with ESMTP id 800859DC81D
	for <pgsql-hackers@postgresql.org>; Thu, 22 Dec 2005 03:01:51 -0400 (AST)
Received: from kleptog by svana.org with local (Exim 3.35 #1 (Debian))
	id 1EpKRg-0005ox-00; Thu, 22 Dec 2005 18:01:00 +1100
Date: Thu, 22 Dec 2005 08:01:00 +0100
From: Martijn van Oosterhout <kleptog@svana.org>
To: Manfred Koizar <mkoi-pg@aon.at>
cc: Tom Lane <tgl@sss.pgh.pa.us>, Dann Corbit <DCorbit@connx.com>,
   Qingqing Zhou <zhouqq@cs.toronto.edu>,
   Bruce Momjian <pgman@candle.pha.pa.us>,
   Luke Lonergan <llonergan@greenplum.com>, Neil Conway <neilc@samurai.com>,
   pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Re: Which qsort is used
Message-ID: <20051222070057.GA21783@svana.org>
Reply-To: Martijn van Oosterhout <kleptog@svana.org>
References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us> <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
	protocol="application/pgp-signature"; boundary="FL5UXtIhxfXey3p5"
Content-Disposition: inline
In-Reply-To: <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com>
User-Agent: Mutt/1.3.28i
X-PGP-Key-ID: Length=1024; ID=0x0DC67BE6
X-PGP-Key-Fingerprint: 295F A899 A81A 156D B522  48A7 6394 F08A 0DC6 7BE6
X-PGP-Key-URL: <http://svana.org/kleptog/0DC67BE6.pgp.asc>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.065 required=5 tests=[AWL=0.065]
X-Spam-Score: 0.065
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR


--FL5UXtIhxfXey3p5
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Thu, Dec 22, 2005 at 01:43:34AM +0100, Manfred Koizar wrote:
> Qsorting N elements costs O(N*lnN), so excluding H elements from the
> sort reduces the cost by at least O(H*lnN).  The merge step costs O(N)
> plus some (<=3D50%) more memory, unless someone knows a fast in-place
> merge.  So depending on the constant factors involved there might be a
> usable solution.

But where are you including the cost to check how many cells are
already sorted? That would be O(H), right? This is where we come back
to the issue that comparisons in PostgreSQL are expensive. The cpu_cost
in the tests I saw so far is unrealistically low.

> I've been playing with some numbers and assuming the constant factors
> to be equal for all the O()'s this method starts to pay off at
> 	  H      for N
> 	  20       100      20%
> 	 130      1000      13%
> 	8000    100000       8%

Hmm, what are the chances you have 100000 unordered items to sort and
that the first 8% will already be in order. ISTM that that probability
will be close enough to zero to not matter...

Have a nice day,
--=20
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

--FL5UXtIhxfXey3p5
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.6 (GNU/Linux)
Comment: For info see http://www.gnupg.org

iD8DBQFDqk8oIB7bNG8LQkwRAjJhAJ47eXRi1DJ02cfKcnN2iPkaBB0eaQCeIiF+
HOAYIPQrU2gpUUiGT3aGUUw=
=R0hU
-----END PGP SIGNATURE-----

--FL5UXtIhxfXey3p5--

From pgsql-hackers-owner+M77831=pgman=candle.pha.pa.us@postgresql.org Thu Dec 22 16:59:19 2005
Return-path: <pgsql-hackers-owner+M77831=pgman=candle.pha.pa.us@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id jBMLxJe07480
	for <pgman@candle.pha.pa.us>; Thu, 22 Dec 2005 16:59:19 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id D1DBE67AC1B
	for <pgman@candle.pha.pa.us>; Thu, 22 Dec 2005 17:59:16 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id BE8249DCBEB
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 22 Dec 2005 17:58:53 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 64765-01
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 22 Dec 2005 17:58:54 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from email.aon.at (warsl404pip7.highway.telekom.at [195.3.96.91])
	by postgresql.org (Postfix) with ESMTP id 3E08E9DCA5C
	for <pgsql-hackers@postgresql.org>; Thu, 22 Dec 2005 17:58:49 -0400 (AST)
Received: (qmail 6986 invoked from network); 22 Dec 2005 21:58:49 -0000
Received: from m150p015.dipool.highway.telekom.at (HELO Sokrates) ([62.46.8.175])
          (envelope-sender <mkoi-pg@aon.at>)
          by smarthub76.highway.telekom.at (qmail-ldap-1.03) with SMTP
          for <kleptog@svana.org>; 22 Dec 2005 21:58:49 -0000
From: Manfred Koizar <mkoi-pg@aon.at>
To: Martijn van Oosterhout <kleptog@svana.org>
cc: Tom Lane <tgl@sss.pgh.pa.us>, Dann Corbit <DCorbit@connx.com>,
   Qingqing Zhou <zhouqq@cs.toronto.edu>,
   Bruce Momjian <pgman@candle.pha.pa.us>,
   Luke Lonergan <llonergan@greenplum.com>, Neil Conway <neilc@samurai.com>,
   pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Re: Which qsort is used
Date: Thu, 22 Dec 2005 22:58:31 +0100
Message-ID: <4r6mq19fe6937mu9130h45ip3oeg135qo3@4ax.com>
References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us> <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com> <20051222070057.GA21783@svana.org>
In-Reply-To: <20051222070057.GA21783@svana.org>
X-Mailer: Forte Agent 3.1/32.783
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.398 required=5 tests=[AWL=0.398]
X-Spam-Score: 0.398
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

On Thu, 22 Dec 2005 08:01:00 +0100, Martijn van Oosterhout
<kleptog@svana.org> wrote:
>But where are you including the cost to check how many cells are
>already sorted? That would be O(H), right?

Yes.  I didn't mention it, because H < N.

> This is where we come back
>to the issue that comparisons in PostgreSQL are expensive.

So we agree that we should try to reduce the number of comparisons.
How many comparisons does it take to sort 100000 items?  1.5 million?

>Hmm, what are the chances you have 100000 unordered items to sort and
>that the first 8% will already be in order. ISTM that that probability
>will be close enough to zero to not matter...

If the items are totally unordered, the check is so cheap you won't
even notice.  OTOH in Tom's example ...

|What I think is much more probable in the Postgres environment
|is almost-but-not-quite-ordered inputs --- eg, a table that was
|perfectly ordered by key when filled, but some of the tuples have since
|been moved by UPDATEs.

... I'd not be surprised if H is 90% of N.
Servus
 Manfred

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

From DCorbit@connx.com Thu Dec 22 17:22:03 2005
Return-path: <DCorbit@connx.com>
Received: from postal.corporate.connx.com (postal.corporate.connx.com [65.212.159.187])
	by candle.pha.pa.us (8.11.6/8.11.6) with SMTP id jBMMLve11671
	for <pgman@candle.pha.pa.us>; Thu, 22 Dec 2005 17:22:03 -0500 (EST)
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Subject: RE: [HACKERS] Re: Which qsort is used
X-MimeOLE: Produced By Microsoft Exchange V6.5
Date: Thu, 22 Dec 2005 14:21:49 -0800
Message-ID: <D425483C2C5C9F49B5B7A41F8944154757D3AC@postal.corporate.connx.com>
Thread-Topic: [HACKERS] Re: Which qsort is used
Thread-Index: AcYHQuXJdKs8JVgmSKywUqld6KYccQAAfWAA
From: "Dann Corbit" <DCorbit@connx.com>
To: "Manfred Koizar" <mkoi-pg@aon.at>,
   "Martijn van Oosterhout" <kleptog@svana.org>
cc: "Tom Lane" <tgl@sss.pgh.pa.us>, "Qingqing Zhou" <zhouqq@cs.toronto.edu>,
   "Bruce Momjian" <pgman@candle.pha.pa.us>,
   "Luke Lonergan" <llonergan@greenplum.com>,
   "Neil Conway" <neilc@samurai.com>, <pgsql-hackers@postgresql.org>
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id jBMMLve11671
Status: OR

An interesting article on sorting and comparison count:
http://www.acm.org/jea/ARTICLES/Vol7Nbr5.pdf

Here is the article, the code, and an implementation that I have been
toying with:
http://cap.connx.com/chess-engines/new-approach/algos.zip

Algorithm quickheap is especially interesting because it does not
require much additional space (just an array of integers up to size
log(element_count) and in addition, it has very few data movements.

> -----Original Message-----
> From: Manfred Koizar [mailto:mkoi-pg@aon.at]
> Sent: Thursday, December 22, 2005 1:59 PM
> To: Martijn van Oosterhout
> Cc: Tom Lane; Dann Corbit; Qingqing Zhou; Bruce Momjian; Luke
Lonergan;
> Neil Conway; pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] Re: Which qsort is used
> 
> On Thu, 22 Dec 2005 08:01:00 +0100, Martijn van Oosterhout
> <kleptog@svana.org> wrote:
> >But where are you including the cost to check how many cells are
> >already sorted? That would be O(H), right?
> 
> Yes.  I didn't mention it, because H < N.
> 
> > This is where we come back
> >to the issue that comparisons in PostgreSQL are expensive.
> 
> So we agree that we should try to reduce the number of comparisons.
> How many comparisons does it take to sort 100000 items?  1.5 million?
> 
> >Hmm, what are the chances you have 100000 unordered items to sort and
> >that the first 8% will already be in order. ISTM that that
probability
> >will be close enough to zero to not matter...
> 
> If the items are totally unordered, the check is so cheap you won't
> even notice.  OTOH in Tom's example ...
> 
> |What I think is much more probable in the Postgres environment
> |is almost-but-not-quite-ordered inputs --- eg, a table that was
> |perfectly ordered by key when filled, but some of the tuples have
since
> |been moved by UPDATEs.
> 
> ... I'd not be surprised if H is 90% of N.
> Servus
>  Manfred

From pgsql-hackers-owner@postgresql.org Mon Dec 19 13:36:58 2005
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 1E0CC9DC810
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Mon, 19 Dec 2005 13:36:58 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 89341-07
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Mon, 19 Dec 2005 13:36:52 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from mail.mi8.com (d01gw02.mi8.com [63.240.6.46])
	by postgresql.org (Postfix) with ESMTP id 348A69DC9C2
	for <pgsql-hackers@postgresql.org>; Mon, 19 Dec 2005 13:36:51 -0400 (AST)
Received: from 172.16.1.25 by mail.mi8.com with ESMTP (- Welcome to Mi8
	Corporation www.Mi8.com (D2)); Mon, 19 Dec 2005 12:36:45 -0500
X-Server-Uuid: 7829E76E-BB9E-4995-8473-3C0929DF7DD1
Received: from MI8NYCMAIL06.Mi8.com ([172.16.1.175]) by
	D01HOST03.Mi8.com with Microsoft SMTPSVC(6.0.3790.1830); Mon, 19 Dec
	2005 12:36:44 -0500
Received: from 67.103.45.218 ([67.103.45.218]) by MI8NYCMAIL06.Mi8.com (
	[172.16.1.219]) via Exchange Front-End Server mi8owa.mi8.com (
	[172.16.1.106]) with Microsoft Exchange Server HTTP-DAV ; Mon, 19 Dec
	2005 17:36:44 +0000
User-Agent: Microsoft-Entourage/11.2.1.051004
Date: Mon, 19 Dec 2005 09:36:44 -0800
Subject: Re: Re: Which qsort is used
From: "Luke Lonergan" <llonergan@greenplum.com>
To: "Martijn van Oosterhout" <kleptog@svana.org>,
	"Dann Corbit" <DCorbit@connx.com>
cc: "Tom Lane" <tgl@sss.pgh.pa.us>,
	"Qingqing Zhou" <zhouqq@cs.toronto.edu>,
	"Bruce Momjian" <pgman@candle.pha.pa.us>,
	"Neil Conway" <neilc@samurai.com>,
	pgsql-hackers@postgresql.org
Message-ID: <BFCC2FAC.16CC0%llonergan@greenplum.com>
Thread-Topic: [HACKERS] Re: Which qsort is used
Thread-Index: AcYEkKvEA7duDr/yQneMyWGCfNr3rQAMhuDl
In-Reply-To: <20051219113724.GD12251@svana.org>
MIME-Version: 1.0
X-OriginalArrivalTime: 19 Dec 2005 17:36:44.0849 (UTC)
	FILETIME=[C7C6AA10:01C604C2]
X-WSS-ID: 6FB830272346940585-01-01
Content-Type: text/plain;
	charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=1.253 required=5 tests=[AWL=0.000,
	RCVD_NUMERIC_HELO=1.253]
X-Spam-Score: 1.253
X-Spam-Level: *
X-Archive-Number: 200512/868
X-Sequence-Number: 77716
Status: OR

Martin,

On 12/19/05 3:37 AM, "Martijn van Oosterhout" <kleptog@svana.org> wrote:

> I'm not sure whether we have a conclusion here, but I do have one
> question: is there a significant difference in the number of times the
> comparison routines are called? Comparisons in PostgreSQL are fairly
> expensive given the fmgr overhead and when comparing tuples it's even
> worse.

It would be interesting to note the comparison count of the different
routines.

Something that really grabbed me about the results though is that the
relative performance of the routines dramatically shifted when the indirect
references in the comparators went in.  The first test I did sorted an array
of int4 - these tests that Qingqing did sorted arrays using an indirect
pointer list, at which point the same distributions performed very
differently.

I suspect that it is the number of comparisons that caused this, and further
that the indirection has disabled the compiler optimizations for memory
prefetch and other things that it could normally recognize.  Given the usage
pattern in Postgres, where sorted things are a mix of strings and intrinsic
types, I'm not sure those optimizations could be done by one routine.

I haven't verified this, but it certainly seems that the NetBSD routine is
the overall winner for the type of use that Postgres has (sorting the using
a pointer list).

- Luke



From pgsql-hackers-owner+M81165@postgresql.org Thu Mar 16 18:37:28 2006
Return-path: <pgsql-hackers-owner+M81165@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2GNbOu11277
	for <pgman@candle.pha.pa.us>; Thu, 16 Mar 2006 18:37:25 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id A609567BADC;
	Thu, 16 Mar 2006 19:37:21 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 8E8E19DC828
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 16 Mar 2006 19:36:50 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 31174-02
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 16 Mar 2006 19:36:52 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id 8CA419DC840
	for <pgsql-hackers@postgresql.org>; Thu, 16 Mar 2006 19:36:46 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2GNagfd023078;
	Thu, 16 Mar 2006 18:36:42 -0500 (EST)
To: "Dann Corbit" <DCorbit@connx.com>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <D425483C2C5C9F49B5B7A41F8944154757D67F@postal.corporate.connx.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D67F@postal.corporate.connx.com>
Comments: In-reply-to "Dann Corbit" <DCorbit@connx.com>
	message dated "Thu, 16 Mar 2006 13:27:33 -0800"
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa0"
Content-ID: <23060.1142551929.0@sss.pgh.pa.us>
Date: Thu, 16 Mar 2006 18:36:42 -0500
Message-ID: <23077.1142552202@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

------- =_aaaaaaaaaa0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <23060.1142551929.1@sss.pgh.pa.us>

>> So at least on randomized data, the swap_cnt thing is a serious loser.
>> Need to run some tests on special-case inputs though.  Anyone have a
>> test suite they like?

> Here is a distribution maker that will create some torture tests for
> sorting programs.

I fleshed out the sort tester that Bentley & McIlroy give pseudocode for
in their paper (attached below in case anyone wants to hack on it).  Not
very surprisingly, it shows the unmodified B&M algorithm as
significantly better than the BSD-lite version:

Our current BSD qsort:
distribution SAWTOOTH: max cratio 12.9259, average 0.870261 over 252 tests
distribution RAND: max cratio 1.07917, average 0.505924 over 252 tests
distribution STAGGER: max cratio 12.9259, average 1.03706 over 252 tests
distribution PLATEAU: max cratio 12.9259, average 0.632514 over 252 tests
distribution SHUFFLE: max cratio 12.9259, average 1.21631 over 252 tests
method COPY: max cratio 3.87533, average 0.666927 over 210 tests
method REVERSE: max cratio 5.6248, average 0.710284 over 210 tests
method FREVERSE: max cratio 12.9259, average 1.58323 over 210 tests
method BREVERSE: max cratio 5.72661, average 1.13674 over 210 tests
method SORT: max cratio 0.758625, average 0.350092 over 210 tests
method DITHER: max cratio 3.13417, average 0.667222 over 210 tests
Overall: average cratio 0.852415 over 1260 tests

without the swap_cnt code:
distribution SAWTOOTH: max cratio 5.6248, average 0.745818 over 252 tests
distribution RAND: max cratio 1.07917, average 0.510097 over 252 tests
distribution STAGGER: max cratio 5.6248, average 1.0494 over 252 tests
distribution PLATEAU: max cratio 3.57655, average 0.411549 over 252 tests
distribution SHUFFLE: max cratio 5.72661, average 1.05988 over 252 tests
method COPY: max cratio 3.87533, average 0.712122 over 210 tests
method REVERSE: max cratio 5.6248, average 0.751011 over 210 tests
method FREVERSE: max cratio 4.80869, average 0.690224 over 210 tests
method BREVERSE: max cratio 5.72661, average 1.13673 over 210 tests
method SORT: max cratio 0.806618, average 0.539829 over 210 tests
method DITHER: max cratio 3.13417, average 0.702174 over 210 tests
Overall: average cratio 0.755348 over 1260 tests

("cratio" is the ratio of the actual number of comparison function calls
to the theoretical expectation of N*lg2(N).)  The insertion sort
switchover is a loser for both average and worst-case measurements.

I tried Dann's distributions too, with N = 100000:

Our current BSD qsort:
dist fib: cratio 0.0694229
dist camel: cratio 0.0903228
dist constant: cratio 0.0602126
dist five: cratio 0.132288
dist ramp: cratio 4.29937
dist random: cratio 1.09286
dist reverse: cratio 0.5663
dist sorted: cratio 0.18062
dist ten: cratio 0.174781
dist twenty: cratio 0.238098
dist two: cratio 0.090365
dist perverse: cratio 0.334503
dist trig: cratio 0.679846
Overall: max cratio 4.29937, average cratio 0.616076 over 13 tests

without the swap_cnt code:
dist fib: cratio 0.0694229
dist camel: cratio 0.0903228
dist constant: cratio 0.0602126
dist five: cratio 0.132288
dist ramp: cratio 4.29937
dist random: cratio 1.09286
dist reverse: cratio 0.89184
dist sorted: cratio 0.884907
dist ten: cratio 0.174781
dist twenty: cratio 0.238098
dist two: cratio 0.090365
dist perverse: cratio 0.334503
dist trig: cratio 0.679846
Overall: max cratio 4.29937, average cratio 0.695293 over 13 tests

In this set of tests the behavior is just about identical, except for
the case of already-sorted input, where the BSD coding runs in O(N)
instead of O(N lg2 N) time.  So that evidently is why some unknown
person put in the special case.

Some further experimentation destroys my original proposal to limit the
size of subfile we'll use the swap_cnt code for: it turns out that that
eliminates the BSD code's advantage for presorted input (at least for
inputs bigger than the limit) without doing anything much in return.

So my feeling is we should just remove the swap_cnt code and return to
the original B&M algorithm.  Being much faster than expected for
presorted input doesn't justify being far slower than expected for
other inputs, IMHO.  In the context of Postgres I doubt that perfectly
sorted input shows up very often anyway.

Comments?

			regards, tom lane


------- =_aaaaaaaaaa0
Content-Type: application/octet-stream
Content-ID: <23060.1142551929.2@sss.pgh.pa.us>
Content-Description: sorttester.c
Content-Transfer-Encoding: base64

I2luY2x1ZGUgPHN0ZGlvLmg+CiNpbmNsdWRlIDxzdGRsaWIuaD4KI2luY2x1
ZGUgPHN0cmluZy5oPgojaW5jbHVkZSA8bWF0aC5oPgoKI2lmZGVmIFVTRV9R
U09SVAojZGVmaW5lIHRlc3RfcXNvcnQgcXNvcnQKI2Vsc2UKZXh0ZXJuIHZv
aWQgdGVzdF9xc29ydCh2b2lkICphLCBzaXplX3Qgbiwgc2l6ZV90IGVzLAoJ
CQkJCSAgIGludCAoKmNtcCkgKGNvbnN0IHZvaWQgKiwgY29uc3Qgdm9pZCAq
KSk7CiNlbmRpZgoKc3RhdGljIGNvbnN0IGludCBuX3ZhbHVlc1tdID0geyAx
MDAsIDEwMjMsIDEwMjQsIDEwMjUgfSA7CiNkZWZpbmUgTUFYX04gMTAyNQoK
dHlwZWRlZiBlbnVtCnsKCVNBV1RPT1RILCBSQU5ELCBTVEFHR0VSLCBQTEFU
RUFVLCBTSFVGRkxFCn0gRElTVDsKI2RlZmluZSBESVNUX0ZJUlNUCVNBV1RP
T1RICiNkZWZpbmUgRElTVF9MQVNUCVNIVUZGTEUKCnN0YXRpYyBjb25zdCBj
aGFyICogY29uc3QgZGlzdG5hbWVbXSA9IHsKCSJTQVdUT09USCIsICJSQU5E
IiwgIlNUQUdHRVIiLCAiUExBVEVBVSIsICJTSFVGRkxFIgp9OwoKdHlwZWRl
ZiBlbnVtCnsKCU1DT1BZLCBNUkVWRVJTRSwgTUZSRVZFUlNFLCBNQlJFVkVS
U0UsIE1TT1JULCBNRElUSEVSCn0gTU9ETUVUSE9EOwojZGVmaW5lIE1FVEhf
RklSU1QJTUNPUFkKI2RlZmluZSBNRVRIX0xBU1QJTURJVEhFUgoKc3RhdGlj
IGNvbnN0IGNoYXIgKiBjb25zdCBtZXRobmFtZVtdID0gewoJIkNPUFkiLCAi
UkVWRVJTRSIsICJGUkVWRVJTRSIsICJCUkVWRVJTRSIsICJTT1JUIiwgIkRJ
VEhFUiIKfTsKCi8qIHBlci10ZXN0IGNvdW50ZXIgKi8Kc3RhdGljIGxvbmcg
bmNvbXBhcmVzOwoKLyogYWNjdW11bGF0ZSByZXN1bHRzIGFjcm9zcyB0ZXN0
cywgZGlzdC13aXNlICovCnN0YXRpYyBkb3VibGUgc3VtY3JhdGlvX2RbRElT
VF9MQVNUKzFdOwpzdGF0aWMgZG91YmxlIG1heGNyYXRpb19kW0RJU1RfTEFT
VCsxXTsKc3RhdGljIGludCBudGVzdHNfZFtESVNUX0xBU1QrMV07Ci8qIGFj
Y3VtdWxhdGUgcmVzdWx0cyBhY3Jvc3MgdGVzdHMsIG1vZC1tZXRob2Qtd2lz
ZSAqLwpzdGF0aWMgZG91YmxlIHN1bWNyYXRpb19tW01FVEhfTEFTVCsxXTsK
c3RhdGljIGRvdWJsZSBtYXhjcmF0aW9fbVtNRVRIX0xBU1QrMV07CnN0YXRp
YyBpbnQgbnRlc3RzX21bTUVUSF9MQVNUKzFdOwoKCnN0YXRpYyBpbnQKaW50
X2NtcChjb25zdCB2b2lkICphLCBjb25zdCB2b2lkICpiKQp7CglpbnQJCWFh
ID0gKihjb25zdCBpbnQgKikgYTsKCWludAkJYmIgPSAqKGNvbnN0IGludCAq
KSBiOwoKCW5jb21wYXJlcysrOwoKCWlmIChhYSA8IGJiKQoJCXJldHVybiAt
MTsKCWlmIChhYSA+IGJiKQoJCXJldHVybiAxOwoJcmV0dXJuIDA7Cn0KCgpz
dGF0aWMgdm9pZAp0ZXN0X2NvbW1vbihESVNUIGRpc3QsIE1PRE1FVEhPRCBt
ZXRoLCB2b2lkICp4dCwgc2l6ZV90IG4sIHNpemVfdCBzeiwKCQkJaW50ICgq
Y21wKSAoY29uc3Qgdm9pZCAqLCBjb25zdCB2b2lkICopKQp7Cglkb3VibGUg
bmxvZ247Cglkb3VibGUgY3JhdGlvOwoKCW5jb21wYXJlcyA9IDA7Cgl0ZXN0
X3Fzb3J0KHh0LCBuLCBzeiwgY21wKTsKCW5sb2duID0gbiAqIGxvZygoZG91
YmxlKSBuKSAvIGxvZygyLjApOwoJY3JhdGlvID0gbmNvbXBhcmVzIC8gbmxv
Z247CglzdW1jcmF0aW9fZFtkaXN0XSArPSBjcmF0aW87CglpZiAoY3JhdGlv
ID4gbWF4Y3JhdGlvX2RbZGlzdF0pCgkJbWF4Y3JhdGlvX2RbZGlzdF0gPSBj
cmF0aW87CgludGVzdHNfZFtkaXN0XSsrOwoJc3VtY3JhdGlvX21bbWV0aF0g
Kz0gY3JhdGlvOwoJaWYgKGNyYXRpbyA+IG1heGNyYXRpb19tW21ldGhdKQoJ
CW1heGNyYXRpb19tW21ldGhdID0gY3JhdGlvOwoJbnRlc3RzX21bbWV0aF0r
KzsKfQoKCi8qIHdvcmsgb24gYSBjb3B5IG9mIHggKi8Kc3RhdGljIHZvaWQK
dGVzdF9pbnRfY29weShESVNUIGRpc3QsIGludCB4W10sIGludCBuKQp7Cglp
bnQJCXh0W01BWF9OXTsKCgltZW1jcHkoeHQsIHgsIG4gKiBzaXplb2YoaW50
KSk7Cgl0ZXN0X2NvbW1vbihkaXN0LCBNQ09QWSwgKHZvaWQgKikgeHQsIG4s
IHNpemVvZihpbnQpLCBpbnRfY21wKTsKfQoKLyogcmV2ZXJzZSB0aGUgcmFu
Z2Ugc3RhcnQgPD0gaSA8IHN0b3AgKi8Kc3RhdGljIHZvaWQKdGVzdF9pbnRf
cmV2ZXJzZShESVNUIGRpc3QsIE1PRE1FVEhPRCBtZXRoLCBpbnQgeFtdLCBp
bnQgbiwgaW50IHN0YXJ0LCBpbnQgc3RvcCkKewoJaW50CQl4dFtNQVhfTl07
CglpbnQJCWk7CgoJbWVtY3B5KHh0LCB4LCBuICogc2l6ZW9mKGludCkpOwoJ
Zm9yIChpID0gc3RhcnQ7IGkgPCBzdG9wOyBpKyspCgl7CgkJeHRbaV0gPSB4
W3N0b3AgLSAxIC0gaV07Cgl9Cgl0ZXN0X2NvbW1vbihkaXN0LCBtZXRoLCAo
dm9pZCAqKSB4dCwgbiwgc2l6ZW9mKGludCksIGludF9jbXApOwp9CgovKiBw
cmUtc29ydCB1c2luZyBhIHRydXN0ZWQgc29ydCAqLwpzdGF0aWMgdm9pZAp0
ZXN0X2ludF9zb3J0KERJU1QgZGlzdCwgaW50IHhbXSwgaW50IG4pCnsKCWlu
dAkJeHRbTUFYX05dOwoKCW1lbWNweSh4dCwgeCwgbiAqIHNpemVvZihpbnQp
KTsKCXFzb3J0KCh2b2lkICopIHh0LCBuLCBzaXplb2YoaW50KSwgaW50X2Nt
cCk7Cgl0ZXN0X2NvbW1vbihkaXN0LCBNU09SVCwgKHZvaWQgKikgeHQsIG4s
IHNpemVvZihpbnQpLCBpbnRfY21wKTsKfQoKLyogYWRkIGklNSB0byB4W2ld
ICovCnN0YXRpYyB2b2lkCnRlc3RfaW50X2RpdGhlcihESVNUIGRpc3QsIGlu
dCB4W10sIGludCBuKQp7CglpbnQJCXh0W01BWF9OXTsKCWludAkJaTsKCglm
b3IgKGkgPSAwOyBpIDwgbjsgaSsrKQoJewoJCXh0W2ldID0geFtpXSArIGkl
NTsKCX0KCXRlc3RfY29tbW9uKGRpc3QsIE1ESVRIRVIsICh2b2lkICopIHh0
LCBuLCBzaXplb2YoaW50KSwgaW50X2NtcCk7Cn0KCgppbnQKbWFpbigpCnsK
CWludAkJeFtNQVhfTl07CglpbnQJCWlfbjsKCWludAkJbjsKCWludAkJbTsK
CURJU1QJZGlzdDsKCU1PRE1FVEhPRCBtZXRoOwoJaW50CQlpOwoJaW50CQlq
OwoJaW50CQlrOwoJZG91YmxlCXN1bWNyYXRpbzsKCWludAkJbnRlc3RzOwoK
CWZvciAoaV9uID0gMDsgaV9uIDwgc2l6ZW9mKG5fdmFsdWVzKS9zaXplb2Yo
bl92YWx1ZXNbMF0pOyBpX24rKykKCXsKCQluID0gbl92YWx1ZXNbaV9uXTsK
CQlmb3IgKG0gPSAxOyBtIDwgMipuOyBtICo9IDIpCgkJewoJCQlmb3IgKGRp
c3QgPSBESVNUX0ZJUlNUOyBkaXN0IDw9IERJU1RfTEFTVDsgZGlzdCsrKQoJ
CQl7CgkJCQlzd2l0Y2ggKGRpc3QpCgkJCQl7CgkJCQkJY2FzZSBTQVdUT09U
SDoKCQkJCQkJZm9yIChpID0gaiA9IDAsIGsgPSAxOyBpIDwgbjsgaSsrKQoJ
CQkJCQl7CgkJCQkJCQl4W2ldID0gaSAlIG07CgkJCQkJCX0KCQkJCQkJYnJl
YWs7CgkJCQkJY2FzZSBSQU5EOgoJCQkJCQlmb3IgKGkgPSBqID0gMCwgayA9
IDE7IGkgPCBuOyBpKyspCgkJCQkJCXsKCQkJCQkJCXhbaV0gPSByYW5kKCkg
JSBtOwoJCQkJCQl9CgkJCQkJCWJyZWFrOwoJCQkJCWNhc2UgU1RBR0dFUjoK
CQkJCQkJZm9yIChpID0gaiA9IDAsIGsgPSAxOyBpIDwgbjsgaSsrKQoJCQkJ
CQl7CgkJCQkJCQl4W2ldID0gKGkqbSArIGkpICUgbjsKCQkJCQkJfQoJCQkJ
CQlicmVhazsKCQkJCQljYXNlIFBMQVRFQVU6CgkJCQkJCWZvciAoaSA9IGog
PSAwLCBrID0gMTsgaSA8IG47IGkrKykKCQkJCQkJewoJCQkJCQkJeFtpXSA9
IGkgPCBtID8gaSA6IG07CgkJCQkJCX0KCQkJCQkJYnJlYWs7CgkJCQkJY2Fz
ZSBTSFVGRkxFOgoJCQkJCQlmb3IgKGkgPSBqID0gMCwgayA9IDE7IGkgPCBu
OyBpKyspCgkJCQkJCXsKCQkJCQkJCXhbaV0gPSAocmFuZCgpJW0pID8gKGor
PTIpIDogKGsrPTIpOwoJCQkJCQl9CgkJCQkJCWJyZWFrOwoJCQkJfQoJCQkJ
dGVzdF9pbnRfY29weShkaXN0LCB4LCBuKTsgLyogd29yayBvbiBhIGNvcHkg
b2YgeCAqLwoJCQkJdGVzdF9pbnRfcmV2ZXJzZShkaXN0LCBNUkVWRVJTRSwg
eCwgbiwgMCwgbik7IC8qIG9uIGEgcmV2ZXJzZWQgY29weSAqLwoJCQkJdGVz
dF9pbnRfcmV2ZXJzZShkaXN0LCBNRlJFVkVSU0UsIHgsIG4sIDAsIG4vMik7
CS8qIGZyb250IGhhbGYgcmV2ZXJzZWQgKi8KCQkJCXRlc3RfaW50X3JldmVy
c2UoZGlzdCwgTUJSRVZFUlNFLCB4LCBuLCBuLzIsIG4pOwkvKiBiYWNrIGhh
bGYgcmV2ZXJzZWQgKi8KCQkJCXRlc3RfaW50X3NvcnQoZGlzdCwgeCwgbik7
IC8qIGFuIG9yZGVyZWQgY29weSAqLwoJCQkJdGVzdF9pbnRfZGl0aGVyKGRp
c3QsIHgsIG4pOyAvKiBhZGQgaSU1IHRvIHhbaV0gKi8KCQkJfQoJCX0KCX0K
Cglmb3IgKGRpc3QgPSBESVNUX0ZJUlNUOyBkaXN0IDw9IERJU1RfTEFTVDsg
ZGlzdCsrKQoJewoJCXByaW50ZigiZGlzdHJpYnV0aW9uICVzOiBtYXggY3Jh
dGlvICVnLCBhdmVyYWdlICVnIG92ZXIgJWQgdGVzdHNcbiIsCgkJCSAgIGRp
c3RuYW1lW2Rpc3RdLAoJCQkgICBtYXhjcmF0aW9fZFtkaXN0XSwKCQkJICAg
c3VtY3JhdGlvX2RbZGlzdF0gLyBudGVzdHNfZFtkaXN0XSwKCQkJICAgbnRl
c3RzX2RbZGlzdF0pOwoJfQoKCXN1bWNyYXRpbyA9IDA7CgludGVzdHMgPSAw
OwoKCWZvciAobWV0aCA9IE1FVEhfRklSU1Q7IG1ldGggPD0gTUVUSF9MQVNU
OyBtZXRoKyspCgl7CgkJcHJpbnRmKCJtZXRob2QgJXM6IG1heCBjcmF0aW8g
JWcsIGF2ZXJhZ2UgJWcgb3ZlciAlZCB0ZXN0c1xuIiwKCQkJICAgbWV0aG5h
bWVbbWV0aF0sCgkJCSAgIG1heGNyYXRpb19tW21ldGhdLAoJCQkgICBzdW1j
cmF0aW9fbVttZXRoXSAvIG50ZXN0c19tW21ldGhdLAoJCQkgICBudGVzdHNf
bVttZXRoXSk7CgkJc3VtY3JhdGlvICs9IHN1bWNyYXRpb19tW21ldGhdOwoJ
CW50ZXN0cyArPSBudGVzdHNfbVttZXRoXTsKCX0KCglwcmludGYoIk92ZXJh
bGw6IGF2ZXJhZ2UgY3JhdGlvICVnIG92ZXIgJWQgdGVzdHNcbiIsCgkJICAg
c3VtY3JhdGlvIC8gbnRlc3RzLAoJCSAgIG50ZXN0cyk7CgoJcmV0dXJuIDA7
Cn0K

------- =_aaaaaaaaaa0
Content-Type: text/plain
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0


---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend

------- =_aaaaaaaaaa0--

From pgsql-hackers-owner+M81167@postgresql.org Thu Mar 16 18:48:37 2006
Return-path: <pgsql-hackers-owner+M81167@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2GNmbu12770
	for <pgman@candle.pha.pa.us>; Thu, 16 Mar 2006 18:48:37 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 570D567BADC;
	Thu, 16 Mar 2006 19:48:35 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id B49219DCBC2
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 16 Mar 2006 19:48:12 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 28142-10
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 16 Mar 2006 19:48:15 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id 9E95A9DCBAD
	for <pgsql-hackers@postgresql.org>; Thu, 16 Mar 2006 19:48:10 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2GNm9Kt023199;
	Thu, 16 Mar 2006 18:48:09 -0500 (EST)
To: Darcy Buskermolen <darcy@wavefire.com>
cc: pgsql-hackers@postgresql.org, "Dann Corbit" <DCorbit@connx.com>,
   "Jonah H. Harris" <jonah.harris@gmail.com>,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <200603161541.25929.darcy@wavefire.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D67D@postal.corporate.connx.com> <19646.1142539750@sss.pgh.pa.us> <200603161541.25929.darcy@wavefire.com>
Comments: In-reply-to Darcy Buskermolen <darcy@wavefire.com>
	message dated "Thu, 16 Mar 2006 15:41:24 -0800"
Date: Thu, 16 Mar 2006 18:48:09 -0500
Message-ID: <23198.1142552889@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

Darcy Buskermolen <darcy@wavefire.com> writes:
> On Thursday 16 March 2006 12:09, Tom Lane wrote:
>> So we still have a problem of software archaeology: who added the
>> insertion sort switch to the NetBSD version, and on what grounds?

> This is when that particular code was pushed in, as to why exactly, you'll 
> have to ask mycroft.
> http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/stdlib/qsort.c.diff?r1=1.3&r2=1.4&only_with_tag=MAIN

Interesting.  It looks to me like he replaced the former
vaguely-Knuth-based coding with B&M's code, but kept the insertion-
sort-after-no-swap special case that was in the previous code.  I'll
betcha he didn't test to see whether this was actually such a great
idea ...

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 1: if posting/reading through Usenet, please send an appropriate
       subscribe-nomail command to majordomo@postgresql.org so that your
       message can get through to the mailing list cleanly

From pgsql-hackers-owner+M81168@postgresql.org Thu Mar 16 19:42:51 2006
Return-path: <pgsql-hackers-owner+M81168@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2H0gpu19805
	for <pgman@candle.pha.pa.us>; Thu, 16 Mar 2006 19:42:51 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 5F20967BADE;
	Thu, 16 Mar 2006 20:42:48 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id AA2239DCBAD
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 16 Mar 2006 20:42:20 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 46728-01
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 16 Mar 2006 20:42:23 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from postal.corporate.connx.com (postal.corporate.connx.com [65.212.159.187])
	by postgresql.org (Postfix) with ESMTP id 062EC9DCBA4
	for <pgsql-hackers@postgresql.org>; Thu, 16 Mar 2006 20:42:17 -0400 (AST)
X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Subject: Re: [HACKERS] qsort, once again 
Date: Thu, 16 Mar 2006 16:42:20 -0800
Message-ID: <D425483C2C5C9F49B5B7A41F8944154757D688@postal.corporate.connx.com>
Thread-Topic: [HACKERS] qsort, once again 
Thread-Index: AcZJUnwKofAzJ+OKTcqF67UTsFWJEQACP7AA
From: "Dann Corbit" <DCorbit@connx.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, <pgsql-hackers@postgresql.org>,
   "Jerry Sievers" <jerry@jerrysievers.com>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.096 required=5 tests=[AWL=0.096]
X-Spam-Score: 0.096
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id k2H0gpu19805
Status: OR

> So my feeling is we should just remove the swap_cnt code and return to
> the original B&M algorithm.  Being much faster than expected for
> presorted input doesn't justify being far slower than expected for
> other inputs, IMHO.  In the context of Postgres I doubt that perfectly
> sorted input shows up very often anyway.
> 
> Comments?

Checking for presorted input is O(n).
If the input is random, an average of 3 elements will be tested.
So adding an in-order check of the data should not be too expensive.

I would benchmark several approaches and see which one is best when used
in-place.


---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?

               http://www.postgresql.org/docs/faq

From pgsql-hackers-owner+M81169@postgresql.org Thu Mar 16 20:13:08 2006
Return-path: <pgsql-hackers-owner+M81169@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2H1D7u25008
	for <pgman@candle.pha.pa.us>; Thu, 16 Mar 2006 20:13:07 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 6B48F67BADE;
	Thu, 16 Mar 2006 21:13:04 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 19FAD9DCC5C
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Thu, 16 Mar 2006 21:12:36 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 53608-01
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Thu, 16 Mar 2006 21:12:37 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from postal.corporate.connx.com (postal.corporate.connx.com [65.212.159.187])
	by postgresql.org (Postfix) with ESMTP id 839069DCC2D
	for <pgsql-hackers@postgresql.org>; Thu, 16 Mar 2006 21:12:32 -0400 (AST)
X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Subject: Re: [HACKERS] qsort, once again 
Date: Thu, 16 Mar 2006 17:12:35 -0800
Message-ID: <D425483C2C5C9F49B5B7A41F8944154757D68B@postal.corporate.connx.com>
Thread-Topic: [HACKERS] qsort, once again 
Thread-Index: AcZJUnwKofAzJ+OKTcqF67UTsFWJEQACP7AAAADU8dA=
From: "Dann Corbit" <DCorbit@connx.com>
To: "Dann Corbit" <DCorbit@connx.com>, "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, <pgsql-hackers@postgresql.org>,
   "Jerry Sievers" <jerry@jerrysievers.com>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.096 required=5 tests=[AWL=0.096]
X-Spam-Score: 0.096
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id k2H1D7u25008
Status: OR

> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-
> owner@postgresql.org] On Behalf Of Dann Corbit
> Sent: Thursday, March 16, 2006 4:42 PM
> To: Tom Lane
> Cc: Jonah H. Harris; pgsql-hackers@postgresql.org; Jerry Sievers
> Subject: Re: [HACKERS] qsort, once again
> 
> > So my feeling is we should just remove the swap_cnt code and return
to
> > the original B&M algorithm.  Being much faster than expected for
> > presorted input doesn't justify being far slower than expected for
> > other inputs, IMHO.  In the context of Postgres I doubt that
perfectly
> > sorted input shows up very often anyway.
> >
> > Comments?
> 
> Checking for presorted input is O(n).
> If the input is random, an average of 3 elements will be tested.
> So adding an in-order check of the data should not be too expensive.
> 
> I would benchmark several approaches and see which one is best when
used
> in-place.

Even if "hunks" of the input are sorted, the test is a very good idea.

Recall that we are sorting recursively and so we divide the data into
chunks.

Consider an example...

Quicksort of a field that contains Sex as 'M' for male, 'F' for female,
or NULL for unknown.

The median selection is going to pick one of 'M', 'F', or NULL.
After pass 1 of qsort we will have two partitions.  One partition will
have all of one type and the other partition will have the other two
types.

An in-order check will tell us that the monotone partition is sorted and
we are done with it.

Imagine also a table that was clustered but for which we have not
updated statistics.  Perhaps it is 98% sorted.  Checking for order in
our partitions is probably a good idea.

I think you could also get a good optimization if you are checking for
partitions and find a big section of the partition is not ordered (even
though the whole thing is not).  If you could perk the ordered size up
the tree, you could just add another partition to the merge list and
sort the unordered part.

In "C Unleashed" I call this idea partition discovery mergesort.

---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

From pgsql-hackers-owner+M81172@postgresql.org Fri Mar 17 00:27:41 2006
Return-path: <pgsql-hackers-owner+M81172@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2H5Reu14258
	for <pgman@candle.pha.pa.us>; Fri, 17 Mar 2006 00:27:40 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id CE86767BAEA;
	Fri, 17 Mar 2006 01:27:36 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 465549DC874
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Fri, 17 Mar 2006 01:27:11 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 76897-01
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Fri, 17 Mar 2006 01:27:10 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id 2456F9DC871
	for <pgsql-hackers@postgresql.org>; Fri, 17 Mar 2006 01:27:08 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2H5R6PF025295;
	Fri, 17 Mar 2006 00:27:06 -0500 (EST)
To: "Dann Corbit" <DCorbit@connx.com>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <D425483C2C5C9F49B5B7A41F8944154757D68B@postal.corporate.connx.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D68B@postal.corporate.connx.com>
Comments: In-reply-to "Dann Corbit" <DCorbit@connx.com>
	message dated "Thu, 16 Mar 2006 17:12:35 -0800"
Date: Fri, 17 Mar 2006 00:27:05 -0500
Message-ID: <25294.1142573225@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

"Dann Corbit" <DCorbit@connx.com> writes:
>> So my feeling is we should just remove the swap_cnt code and return
>> to the original B&M algorithm.

> Even if "hunks" of the input are sorted, the test is a very good idea.

Yah know, guys, Bentley and McIlroy are each smarter than any five of
us, and I'm quite certain it occurred to them to try prechecking for
sorted input.  If that case is not in their code then it's probably
because it's a net loss.  Unless you have reason to think that sorted
input is *more* common than other cases for the Postgres environment,
which is certainly a fact not in evidence.

(Bentley was my thesis adviser for awhile before he went to Bell Labs,
so my respect for him is based on direct personal experience.  McIlroy
I only know by reputation, but he's sure got a ton of that.)

> Imagine also a table that was clustered but for which we have not
> updated statistics.  Perhaps it is 98% sorted.  Checking for order in
> our partitions is probably a good idea.

If we are using the sort code rather than the recently-clustered index
for such a case, then we have problems elsewhere.  This scenario is not
a good argument that the sort code needs to be specialized to handle
this case at the expense of other cases; the place to be fixing it is
the planner or the statistics-management code.

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not
       match

From pgsql-hackers-owner+M81173@postgresql.org Fri Mar 17 00:29:24 2006
Return-path: <pgsql-hackers-owner+M81173@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2H5TNu14505
	for <pgman@candle.pha.pa.us>; Fri, 17 Mar 2006 00:29:23 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id A271D67BAEA;
	Fri, 17 Mar 2006 01:29:19 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id C96D79DCA40
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Fri, 17 Mar 2006 01:28:55 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 44062-07
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Fri, 17 Mar 2006 01:28:54 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from postal.corporate.connx.com (postal.corporate.connx.com [65.212.159.187])
	by postgresql.org (Postfix) with ESMTP id CDE6C9DCA4B
	for <pgsql-hackers@postgresql.org>; Fri, 17 Mar 2006 01:28:53 -0400 (AST)
X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: text/plain;
	charset="us-ascii"
Subject: Re: [HACKERS] qsort, once again 
Date: Thu, 16 Mar 2006 21:28:52 -0800
Message-ID: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com>
Thread-Topic: [HACKERS] qsort, once again 
Thread-Index: AcZJg2/Nvc2IdeFUT0id8WtNczZvGQAACfYw
From: "Dann Corbit" <DCorbit@connx.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, <pgsql-hackers@postgresql.org>,
   "Jerry Sievers" <jerry@jerrysievers.com>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.097 required=5 tests=[AWL=0.097]
X-Spam-Score: 0.097
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by candle.pha.pa.us id k2H5TNu14505
Status: OR

Well, my point was that it is a snap to implement and test.

It will be better, worse, or the same.

I agree that Bentley is a bloody genius.

> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: Thursday, March 16, 2006 9:27 PM
> To: Dann Corbit
> Cc: Jonah H. Harris; pgsql-hackers@postgresql.org; Jerry Sievers
> Subject: Re: [HACKERS] qsort, once again
> 
> "Dann Corbit" <DCorbit@connx.com> writes:
> >> So my feeling is we should just remove the swap_cnt code and return
> >> to the original B&M algorithm.
> 
> > Even if "hunks" of the input are sorted, the test is a very good
idea.
> 
> Yah know, guys, Bentley and McIlroy are each smarter than any five of
> us, and I'm quite certain it occurred to them to try prechecking for
> sorted input.  If that case is not in their code then it's probably
> because it's a net loss.  Unless you have reason to think that sorted
> input is *more* common than other cases for the Postgres environment,
> which is certainly a fact not in evidence.
> 
> (Bentley was my thesis adviser for awhile before he went to Bell Labs,
> so my respect for him is based on direct personal experience.  McIlroy
> I only know by reputation, but he's sure got a ton of that.)
> 
> > Imagine also a table that was clustered but for which we have not
> > updated statistics.  Perhaps it is 98% sorted.  Checking for order
in
> > our partitions is probably a good idea.
> 
> If we are using the sort code rather than the recently-clustered index
> for such a case, then we have problems elsewhere.  This scenario is
not
> a good argument that the sort code needs to be specialized to handle
> this case at the expense of other cases; the place to be fixing it is
> the planner or the statistics-management code.
> 
> 			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster

From pgsql-hackers-owner+M81277@postgresql.org Tue Mar 21 13:53:08 2006
Return-path: <pgsql-hackers-owner+M81277@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2LIr6M03797
	for <pgman@candle.pha.pa.us>; Tue, 21 Mar 2006 13:53:06 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 01A8467BBF2;
	Tue, 21 Mar 2006 14:53:00 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 0ED1D9DCCD2
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Tue, 21 Mar 2006 14:52:29 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 38232-04-3
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Tue, 21 Mar 2006 14:52:26 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id 1A0EE9DCC2C
	for <pgsql-hackers@postgresql.org>; Tue, 21 Mar 2006 14:52:22 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2LIqHPc018733;
	Tue, 21 Mar 2006 13:52:17 -0500 (EST)
To: "Dann Corbit" <DCorbit@connx.com>
cc: "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com>
Comments: In-reply-to "Dann Corbit" <DCorbit@connx.com>
	message dated "Thu, 16 Mar 2006 21:28:52 -0800"
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa0"
Content-ID: <18685.1142966822.0@sss.pgh.pa.us>
Date: Tue, 21 Mar 2006 13:52:17 -0500
Message-ID: <18732.1142967137@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

------- =_aaaaaaaaaa0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <18685.1142966822.1@sss.pgh.pa.us>

"Dann Corbit" <DCorbit@connx.com> writes:
> Well, my point was that it is a snap to implement and test.

Well, having done this, I have to eat my words: it does seem to be a
pretty good idea.

The following test numbers are using Bentley & McIlroy's test framework,
but modified to test only the case N=10000 rather than the four smaller
N values they originally used.  I did that because it exposes quadratic
behavior more obviously, and the variance in N made it harder to compare
comparison ratios for different cases.  I also added a "NEARSORT" test
method, which sorts the input distribution and then exchanges two
elements chosen at random.  I did that because I was concerned that
nearly sorted input would be the worst case for the presorted-input
check, as it would waste the most cycles before failing on such input.

With our existing qsort code, the results look like

distribution SAWTOOTH: max cratio 94.17, min 0.08, average 1.56 over 105 tests
distribution RAND: max cratio 1.06, min 0.08, average 0.51 over 105 tests
distribution STAGGER: max cratio 6.08, min 0.23, average 1.01 over 105 tests
distribution PLATEAU: max cratio 94.17, min 0.08, average 2.12 over 105 tests
distribution SHUFFLE: max cratio 94.17, min 0.23, average 1.92 over 105 tests
method COPY: max cratio 6.08, min 0.08, average 0.72 over 75 tests
method REVERSE: max cratio 5.34, min 0.08, average 0.69 over 75 tests
method FREVERSE: max cratio 94.17, min 0.08, average 5.71 over 75 tests
method BREVERSE: max cratio 3.86, min 0.08, average 1.41 over 75 tests
method SORT: max cratio 0.82, min 0.08, average 0.31 over 75 tests
method NEARSORT: max cratio 0.82, min 0.08, average 0.36 over 75 tests
method DITHER: max cratio 5.52, min 0.18, average 0.77 over 75 tests
Overall: average cratio 1.42 over 525 tests

("cratio" is the ratio of the actual number of comparison function calls
to the theoretical expectation, N log2(N))

That's pretty awful: there are several test cases that make it use
nearly 100 times the expected number of comparisons.

Removing the swap_cnt test to bring it close to B&M's original
recommendations, we get

distribution SAWTOOTH: max cratio 3.85, min 0.08, average 0.70 over 105 tests
distribution RAND: max cratio 1.06, min 0.08, average 0.52 over 105 tests
distribution STAGGER: max cratio 6.08, min 0.58, average 1.12 over 105 tests
distribution PLATEAU: max cratio 3.70, min 0.08, average 0.34 over 105 tests
distribution SHUFFLE: max cratio 3.86, min 0.86, average 1.24 over 105 tests
method COPY: max cratio 6.08, min 0.08, average 0.76 over 75 tests
method REVERSE: max cratio 5.34, min 0.08, average 0.75 over 75 tests
method FREVERSE: max cratio 4.56, min 0.08, average 0.73 over 75 tests
method BREVERSE: max cratio 3.86, min 0.08, average 1.41 over 75 tests
method SORT: max cratio 0.86, min 0.08, average 0.56 over 75 tests
method NEARSORT: max cratio 0.86, min 0.08, average 0.56 over 75 tests
method DITHER: max cratio 3.73, min 0.18, average 0.72 over 75 tests
Overall: average cratio 0.78 over 525 tests

which is a whole lot better as to both average and worst cases.

I then added some code to check for presorted input (just after the
n<7 insertion sort code):

#ifdef CHECK_SORTED
	presorted = 1;
	for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
	{
		if (cmp(pm - es, pm) > 0)
		{
			presorted = 0;
			break;
		}
	}
	if (presorted)
		return;
#endif

This gives

distribution SAWTOOTH: max cratio 3.88, min 0.08, average 0.62 over 105 tests
distribution RAND: max cratio 1.06, min 0.08, average 0.46 over 105 tests
distribution STAGGER: max cratio 6.15, min 0.08, average 0.98 over 105 tests
distribution PLATEAU: max cratio 3.79, min 0.08, average 0.31 over 105 tests
distribution SHUFFLE: max cratio 3.91, min 0.08, average 1.09 over 105 tests
method COPY: max cratio 6.15, min 0.08, average 0.72 over 75 tests
method REVERSE: max cratio 5.34, min 0.08, average 0.76 over 75 tests
method FREVERSE: max cratio 4.58, min 0.08, average 0.73 over 75 tests
method BREVERSE: max cratio 3.91, min 0.08, average 1.44 over 75 tests
method SORT: max cratio 0.08, min 0.08, average 0.08 over 75 tests
method NEARSORT: max cratio 0.89, min 0.08, average 0.39 over 75 tests
method DITHER: max cratio 3.73, min 0.18, average 0.72 over 75 tests
Overall: average cratio 0.69 over 525 tests

So the worst case seems only very marginally worse, and there is a
definite improvement in the average case, even for inputs that aren't
entirely sorted.  Importantly, the "near sorted" case that I thought
might send it into quadratic behavior doesn't seem to do that.

So, unless anyone wants to do further testing, I'll go ahead and commit
these changes.

			regards, tom lane

PS: Just as a comparison point, here are the results when testing HPUX's
library qsort:

distribution SAWTOOTH: max cratio 7.00, min 0.08, average 0.76 over 105 tests
distribution RAND: max cratio 1.11, min 0.08, average 0.53 over 105 tests
distribution STAGGER: max cratio 7.05, min 0.58, average 1.24 over 105 tests
distribution PLATEAU: max cratio 7.00, min 0.08, average 0.43 over 105 tests
distribution SHUFFLE: max cratio 7.00, min 0.86, average 1.54 over 105 tests
method COPY: max cratio 6.70, min 0.08, average 0.79 over 75 tests
method REVERSE: max cratio 7.05, min 0.08, average 0.78 over 75 tests
method FREVERSE: max cratio 7.00, min 0.08, average 0.77 over 75 tests
method BREVERSE: max cratio 7.00, min 0.08, average 2.11 over 75 tests
method SORT: max cratio 0.86, min 0.08, average 0.56 over 75 tests
method NEARSORT: max cratio 0.86, min 0.08, average 0.56 over 75 tests
method DITHER: max cratio 4.06, min 0.16, average 0.74 over 75 tests
Overall: average cratio 0.90 over 525 tests

and here are the results using glibc's qsort, which of course isn't
quicksort at all but some kind of merge sort:

distribution SAWTOOTH: max cratio 0.90, min 0.49, average 0.65 over 105 tests
distribution RAND: max cratio 0.91, min 0.49, average 0.76 over 105 tests
distribution STAGGER: max cratio 0.92, min 0.49, average 0.70 over 105 tests
distribution PLATEAU: max cratio 0.84, min 0.49, average 0.54 over 105 tests
distribution SHUFFLE: max cratio 0.64, min 0.49, average 0.52 over 105 tests
method COPY: max cratio 0.92, min 0.49, average 0.66 over 75 tests
method REVERSE: max cratio 0.92, min 0.49, average 0.68 over 75 tests
method FREVERSE: max cratio 0.92, min 0.49, average 0.67 over 75 tests
method BREVERSE: max cratio 0.92, min 0.49, average 0.68 over 75 tests
method SORT: max cratio 0.49, min 0.49, average 0.49 over 75 tests
method NEARSORT: max cratio 0.55, min 0.49, average 0.51 over 75 tests
method DITHER: max cratio 0.92, min 0.50, average 0.74 over 75 tests
Overall: average cratio 0.63 over 525 tests

PPS: final version of test framework attached for the archives.


------- =_aaaaaaaaaa0
Content-Type: application/octet-stream
Content-ID: <18685.1142966822.2@sss.pgh.pa.us>
Content-Description: sorttester.c
Content-Transfer-Encoding: base64

I2luY2x1ZGUgPHN0ZGlvLmg+CiNpbmNsdWRlIDxzdGRsaWIuaD4KI2luY2x1
ZGUgPHN0cmluZy5oPgojaW5jbHVkZSA8bWF0aC5oPgoKI2lmZGVmIFVTRV9R
U09SVAojZGVmaW5lIHRlc3RfcXNvcnQgcXNvcnQKI2Vsc2UKZXh0ZXJuIHZv
aWQgdGVzdF9xc29ydCh2b2lkICphLCBzaXplX3Qgbiwgc2l6ZV90IGVzLAoJ
CQkJCSAgIGludCAoKmNtcCkgKGNvbnN0IHZvaWQgKiwgY29uc3Qgdm9pZCAq
KSk7CiNlbmRpZgoKLy9zdGF0aWMgY29uc3QgaW50IG5fdmFsdWVzW10gPSB7
IDEwMCwgMTAyMywgMTAyNCwgMTAyNSwgMTAwMDAgfSA7CnN0YXRpYyBjb25z
dCBpbnQgbl92YWx1ZXNbXSA9IHsgMTAwMDAgfSA7CiNkZWZpbmUgTUFYX04g
MTAwMDAKCnR5cGVkZWYgZW51bQp7CglTQVdUT09USCwgUkFORCwgU1RBR0dF
UiwgUExBVEVBVSwgU0hVRkZMRQp9IERJU1Q7CiNkZWZpbmUgRElTVF9GSVJT
VAlTQVdUT09USAojZGVmaW5lIERJU1RfTEFTVAlTSFVGRkxFCgpzdGF0aWMg
Y29uc3QgY2hhciAqIGNvbnN0IGRpc3RuYW1lW10gPSB7CgkiU0FXVE9PVEgi
LCAiUkFORCIsICJTVEFHR0VSIiwgIlBMQVRFQVUiLCAiU0hVRkZMRSIKfTsK
CnR5cGVkZWYgZW51bQp7CglNQ09QWSwgTVJFVkVSU0UsIE1GUkVWRVJTRSwg
TUJSRVZFUlNFLCBNU09SVCwgTU5FQVJTT1JULCBNRElUSEVSCn0gTU9ETUVU
SE9EOwojZGVmaW5lIE1FVEhfRklSU1QJTUNPUFkKI2RlZmluZSBNRVRIX0xB
U1QJTURJVEhFUgoKc3RhdGljIGNvbnN0IGNoYXIgKiBjb25zdCBtZXRobmFt
ZVtdID0gewoJIkNPUFkiLCAiUkVWRVJTRSIsICJGUkVWRVJTRSIsICJCUkVW
RVJTRSIsICJTT1JUIiwgIk5FQVJTT1JUIiwgIkRJVEhFUiIKfTsKCi8qIHBl
ci10ZXN0IGNvdW50ZXIgKi8Kc3RhdGljIGxvbmcgbmNvbXBhcmVzOwoKLyog
YWNjdW11bGF0ZSByZXN1bHRzIGFjcm9zcyB0ZXN0cywgZGlzdC13aXNlICov
CnN0YXRpYyBkb3VibGUgc3VtY3JhdGlvX2RbRElTVF9MQVNUKzFdOwpzdGF0
aWMgZG91YmxlIG1heGNyYXRpb19kW0RJU1RfTEFTVCsxXTsKc3RhdGljIGRv
dWJsZSBtaW5jcmF0aW9fZFtESVNUX0xBU1QrMV07CnN0YXRpYyBpbnQgbnRl
c3RzX2RbRElTVF9MQVNUKzFdOwovKiBhY2N1bXVsYXRlIHJlc3VsdHMgYWNy
b3NzIHRlc3RzLCBtb2QtbWV0aG9kLXdpc2UgKi8Kc3RhdGljIGRvdWJsZSBz
dW1jcmF0aW9fbVtNRVRIX0xBU1QrMV07CnN0YXRpYyBkb3VibGUgbWF4Y3Jh
dGlvX21bTUVUSF9MQVNUKzFdOwpzdGF0aWMgZG91YmxlIG1pbmNyYXRpb19t
W01FVEhfTEFTVCsxXTsKc3RhdGljIGludCBudGVzdHNfbVtNRVRIX0xBU1Qr
MV07CgoKc3RhdGljIGludAppbnRfY21wKGNvbnN0IHZvaWQgKmEsIGNvbnN0
IHZvaWQgKmIpCnsKCWludAkJYWEgPSAqKGNvbnN0IGludCAqKSBhOwoJaW50
CQliYiA9ICooY29uc3QgaW50ICopIGI7CgoJbmNvbXBhcmVzKys7CgoJaWYg
KGFhIDwgYmIpCgkJcmV0dXJuIC0xOwoJaWYgKGFhID4gYmIpCgkJcmV0dXJu
IDE7CglyZXR1cm4gMDsKfQoKCnN0YXRpYyB2b2lkCnRlc3RfY29tbW9uKERJ
U1QgZGlzdCwgTU9ETUVUSE9EIG1ldGgsIHZvaWQgKnh0LCBzaXplX3Qgbiwg
c2l6ZV90IHN6LAoJCQlpbnQgKCpjbXApIChjb25zdCB2b2lkICosIGNvbnN0
IHZvaWQgKikpCnsKCWRvdWJsZSBubG9nbjsKCWRvdWJsZSBjcmF0aW87CgoJ
bmNvbXBhcmVzID0gMDsKCXRlc3RfcXNvcnQoeHQsIG4sIHN6LCBjbXApOwoJ
LyogbG9nIHRoZSBjb3N0IGJlZm9yZSBkb2luZyBtb3JlIGNtcHMgKi8KCW5s
b2duID0gbiAqIGxvZygoZG91YmxlKSBuKSAvIGxvZygyLjApOwoJY3JhdGlv
ID0gbmNvbXBhcmVzIC8gbmxvZ247CglzdW1jcmF0aW9fZFtkaXN0XSArPSBj
cmF0aW87CglpZiAobnRlc3RzX2RbZGlzdF0gPT0gMCkKCXsKCQltYXhjcmF0
aW9fZFtkaXN0XSA9IG1pbmNyYXRpb19kW2Rpc3RdID0gY3JhdGlvOwoJfQoJ
ZWxzZQoJewoJCWlmIChjcmF0aW8gPiBtYXhjcmF0aW9fZFtkaXN0XSkKCQkJ
bWF4Y3JhdGlvX2RbZGlzdF0gPSBjcmF0aW87CgkJaWYgKGNyYXRpbyA8IG1p
bmNyYXRpb19kW2Rpc3RdKQoJCQltaW5jcmF0aW9fZFtkaXN0XSA9IGNyYXRp
bzsKCX0KCW50ZXN0c19kW2Rpc3RdKys7CglzdW1jcmF0aW9fbVttZXRoXSAr
PSBjcmF0aW87CglpZiAobnRlc3RzX21bbWV0aF0gPT0gMCkKCXsKCQltYXhj
cmF0aW9fbVttZXRoXSA9IG1pbmNyYXRpb19tW21ldGhdID0gY3JhdGlvOwoJ
fQoJZWxzZQoJewoJCWlmIChjcmF0aW8gPiBtYXhjcmF0aW9fbVttZXRoXSkK
CQkJbWF4Y3JhdGlvX21bbWV0aF0gPSBjcmF0aW87CgkJaWYgKGNyYXRpbyA8
IG1pbmNyYXRpb19tW21ldGhdKQoJCQltaW5jcmF0aW9fbVttZXRoXSA9IGNy
YXRpbzsKCX0KCW50ZXN0c19tW21ldGhdKys7CgkvKiBub3cgY2hlY2sgZm9y
IGNvcnJlY3Qgc29ydGluZyAqLwoJewoJCWNoYXIgKnAgPSB4dDsKCQl3aGls
ZSAobi0tID4gMSkKCQl7CgkJCWlmIChjbXAocCwgcCArIHN6KSA+IDApCgkJ
CXsKCQkJCWZwcmludGYoc3RkZXJyLCAid3Jvbmcgc29ydCByZXN1bHQgZm9y
ICVzLyVzIVxuIiwKCQkJCQkJZGlzdG5hbWVbZGlzdF0sIG1ldGhuYW1lW21l
dGhdKTsKCQkJCWV4aXQoMSk7CgkJCX0KCQkJcCArPSBzejsKCQl9Cgl9Cn0K
CgovKiB3b3JrIG9uIGEgY29weSBvZiB4ICovCnN0YXRpYyB2b2lkCnRlc3Rf
aW50X2NvcHkoRElTVCBkaXN0LCBpbnQgeFtdLCBpbnQgbikKewoJaW50CQl4
dFtNQVhfTl07CgoJbWVtY3B5KHh0LCB4LCBuICogc2l6ZW9mKGludCkpOwoJ
dGVzdF9jb21tb24oZGlzdCwgTUNPUFksICh2b2lkICopIHh0LCBuLCBzaXpl
b2YoaW50KSwgaW50X2NtcCk7Cn0KCi8qIHJldmVyc2UgdGhlIHJhbmdlIHN0
YXJ0IDw9IGkgPCBzdG9wICovCnN0YXRpYyB2b2lkCnRlc3RfaW50X3JldmVy
c2UoRElTVCBkaXN0LCBNT0RNRVRIT0QgbWV0aCwgaW50IHhbXSwgaW50IG4s
IGludCBzdGFydCwgaW50IHN0b3ApCnsKCWludAkJeHRbTUFYX05dOwoJaW50
CQlpOwoKCW1lbWNweSh4dCwgeCwgbiAqIHNpemVvZihpbnQpKTsKCWZvciAo
aSA9IHN0YXJ0OyBpIDwgc3RvcDsgaSsrKQoJewoJCXh0W2ldID0geFtzdG9w
IC0gMSAtIGldOwoJfQoJdGVzdF9jb21tb24oZGlzdCwgbWV0aCwgKHZvaWQg
KikgeHQsIG4sIHNpemVvZihpbnQpLCBpbnRfY21wKTsKfQoKLyogcHJlLXNv
cnQgdXNpbmcgYSB0cnVzdGVkIHNvcnQgKi8Kc3RhdGljIHZvaWQKdGVzdF9p
bnRfc29ydChESVNUIGRpc3QsIGludCB4W10sIGludCBuKQp7CglpbnQJCXh0
W01BWF9OXTsKCgltZW1jcHkoeHQsIHgsIG4gKiBzaXplb2YoaW50KSk7Cglx
c29ydCgodm9pZCAqKSB4dCwgbiwgc2l6ZW9mKGludCksIGludF9jbXApOwoJ
dGVzdF9jb21tb24oZGlzdCwgTVNPUlQsICh2b2lkICopIHh0LCBuLCBzaXpl
b2YoaW50KSwgaW50X2NtcCk7Cn0KCi8qIG5lYXItc29ydGVkICovCnN0YXRp
YyB2b2lkCnRlc3RfaW50X25lYXJzb3J0KERJU1QgZGlzdCwgaW50IHhbXSwg
aW50IG4pCnsKCWludAkJeHRbTUFYX05dOwoKCW1lbWNweSh4dCwgeCwgbiAq
IHNpemVvZihpbnQpKTsKCXFzb3J0KCh2b2lkICopIHh0LCBuLCBzaXplb2Yo
aW50KSwgaW50X2NtcCk7CgkvKiBzd2FwIGEgcmFuZG9tIHR3byBlbGVtZW50
cyAqLwoJewoJCWludCBpID0gcmFuZCgpICUgbjsKCQlpbnQgaiA9IHJhbmQo
KSAlIG47CgkJaW50IHQgPSB4dFtpXTsKCQl4dFtpXSA9IHh0W2pdOwoJCXh0
W2pdID0gdDsKCX0KCXRlc3RfY29tbW9uKGRpc3QsIE1ORUFSU09SVCwgKHZv
aWQgKikgeHQsIG4sIHNpemVvZihpbnQpLCBpbnRfY21wKTsKfQoKLyogYWRk
IGklNSB0byB4W2ldICovCnN0YXRpYyB2b2lkCnRlc3RfaW50X2RpdGhlcihE
SVNUIGRpc3QsIGludCB4W10sIGludCBuKQp7CglpbnQJCXh0W01BWF9OXTsK
CWludAkJaTsKCglmb3IgKGkgPSAwOyBpIDwgbjsgaSsrKQoJewoJCXh0W2ld
ID0geFtpXSArIGklNTsKCX0KCXRlc3RfY29tbW9uKGRpc3QsIE1ESVRIRVIs
ICh2b2lkICopIHh0LCBuLCBzaXplb2YoaW50KSwgaW50X2NtcCk7Cn0KCgpp
bnQKbWFpbigpCnsKCWludAkJeFtNQVhfTl07CglpbnQJCWlfbjsKCWludAkJ
bjsKCWludAkJbTsKCURJU1QJZGlzdDsKCU1PRE1FVEhPRCBtZXRoOwoJaW50
CQlpOwoJaW50CQlqOwoJaW50CQlrOwoJZG91YmxlCXN1bWNyYXRpbzsKCWlu
dAkJbnRlc3RzOwoKCWZvciAoaV9uID0gMDsgaV9uIDwgc2l6ZW9mKG5fdmFs
dWVzKS9zaXplb2Yobl92YWx1ZXNbMF0pOyBpX24rKykKCXsKCQluID0gbl92
YWx1ZXNbaV9uXTsKCQlmb3IgKG0gPSAxOyBtIDwgMipuOyBtICo9IDIpCgkJ
ewoJCQlmb3IgKGRpc3QgPSBESVNUX0ZJUlNUOyBkaXN0IDw9IERJU1RfTEFT
VDsgZGlzdCsrKQoJCQl7CgkJCQlzd2l0Y2ggKGRpc3QpCgkJCQl7CgkJCQkJ
Y2FzZSBTQVdUT09USDoKCQkJCQkJZm9yIChpID0gaiA9IDAsIGsgPSAxOyBp
IDwgbjsgaSsrKQoJCQkJCQl7CgkJCQkJCQl4W2ldID0gaSAlIG07CgkJCQkJ
CX0KCQkJCQkJYnJlYWs7CgkJCQkJY2FzZSBSQU5EOgoJCQkJCQlmb3IgKGkg
PSBqID0gMCwgayA9IDE7IGkgPCBuOyBpKyspCgkJCQkJCXsKCQkJCQkJCXhb
aV0gPSByYW5kKCkgJSBtOwoJCQkJCQl9CgkJCQkJCWJyZWFrOwoJCQkJCWNh
c2UgU1RBR0dFUjoKCQkJCQkJZm9yIChpID0gaiA9IDAsIGsgPSAxOyBpIDwg
bjsgaSsrKQoJCQkJCQl7CgkJCQkJCQl4W2ldID0gKGkqbSArIGkpICUgbjsK
CQkJCQkJfQoJCQkJCQlicmVhazsKCQkJCQljYXNlIFBMQVRFQVU6CgkJCQkJ
CWZvciAoaSA9IGogPSAwLCBrID0gMTsgaSA8IG47IGkrKykKCQkJCQkJewoJ
CQkJCQkJeFtpXSA9IGkgPCBtID8gaSA6IG07CgkJCQkJCX0KCQkJCQkJYnJl
YWs7CgkJCQkJY2FzZSBTSFVGRkxFOgoJCQkJCQlmb3IgKGkgPSBqID0gMCwg
ayA9IDE7IGkgPCBuOyBpKyspCgkJCQkJCXsKCQkJCQkJCXhbaV0gPSAocmFu
ZCgpJW0pID8gKGorPTIpIDogKGsrPTIpOwoJCQkJCQl9CgkJCQkJCWJyZWFr
OwoJCQkJfQoJCQkJdGVzdF9pbnRfY29weShkaXN0LCB4LCBuKTsgLyogd29y
ayBvbiBhIGNvcHkgb2YgeCAqLwoJCQkJdGVzdF9pbnRfcmV2ZXJzZShkaXN0
LCBNUkVWRVJTRSwgeCwgbiwgMCwgbik7IC8qIG9uIGEgcmV2ZXJzZWQgY29w
eSAqLwoJCQkJdGVzdF9pbnRfcmV2ZXJzZShkaXN0LCBNRlJFVkVSU0UsIHgs
IG4sIDAsIG4vMik7CS8qIGZyb250IGhhbGYgcmV2ZXJzZWQgKi8KCQkJCXRl
c3RfaW50X3JldmVyc2UoZGlzdCwgTUJSRVZFUlNFLCB4LCBuLCBuLzIsIG4p
OwkvKiBiYWNrIGhhbGYgcmV2ZXJzZWQgKi8KCQkJCXRlc3RfaW50X3NvcnQo
ZGlzdCwgeCwgbik7IC8qIGFuIG9yZGVyZWQgY29weSAqLwoJCQkJdGVzdF9p
bnRfbmVhcnNvcnQoZGlzdCwgeCwgbik7IC8qIGFuIGFsbW9zdCBvcmRlcmVk
IGNvcHkgKi8KCQkJCXRlc3RfaW50X2RpdGhlcihkaXN0LCB4LCBuKTsgLyog
YWRkIGklNSB0byB4W2ldICovCgkJCX0KCQl9Cgl9CgoJZm9yIChkaXN0ID0g
RElTVF9GSVJTVDsgZGlzdCA8PSBESVNUX0xBU1Q7IGRpc3QrKykKCXsKCQlw
cmludGYoImRpc3RyaWJ1dGlvbiAlczogbWF4IGNyYXRpbyAlLjJmLCBtaW4g
JS4yZiwgYXZlcmFnZSAlLjJmIG92ZXIgJWQgdGVzdHNcbiIsCgkJCSAgIGRp
c3RuYW1lW2Rpc3RdLAoJCQkgICBtYXhjcmF0aW9fZFtkaXN0XSwKCQkJICAg
bWluY3JhdGlvX2RbZGlzdF0sCgkJCSAgIHN1bWNyYXRpb19kW2Rpc3RdIC8g
bnRlc3RzX2RbZGlzdF0sCgkJCSAgIG50ZXN0c19kW2Rpc3RdKTsKCX0KCglz
dW1jcmF0aW8gPSAwOwoJbnRlc3RzID0gMDsKCglmb3IgKG1ldGggPSBNRVRI
X0ZJUlNUOyBtZXRoIDw9IE1FVEhfTEFTVDsgbWV0aCsrKQoJewoJCXByaW50
ZigibWV0aG9kICVzOiBtYXggY3JhdGlvICUuMmYsIG1pbiAlLjJmLCBhdmVy
YWdlICUuMmYgb3ZlciAlZCB0ZXN0c1xuIiwKCQkJICAgbWV0aG5hbWVbbWV0
aF0sCgkJCSAgIG1heGNyYXRpb19tW21ldGhdLAoJCQkgICBtaW5jcmF0aW9f
bVttZXRoXSwKCQkJICAgc3VtY3JhdGlvX21bbWV0aF0gLyBudGVzdHNfbVtt
ZXRoXSwKCQkJICAgbnRlc3RzX21bbWV0aF0pOwoJCXN1bWNyYXRpbyArPSBz
dW1jcmF0aW9fbVttZXRoXTsKCQludGVzdHMgKz0gbnRlc3RzX21bbWV0aF07
Cgl9CgoJcHJpbnRmKCJPdmVyYWxsOiBhdmVyYWdlIGNyYXRpbyAlLjJmIG92
ZXIgJWQgdGVzdHNcbiIsCgkJICAgc3VtY3JhdGlvIC8gbnRlc3RzLAoJCSAg
IG50ZXN0cyk7CgoJcmV0dXJuIDA7Cn0K

------- =_aaaaaaaaaa0
Content-Type: text/plain
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0


---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

------- =_aaaaaaaaaa0--

From pgsql-hackers-owner+M81283@postgresql.org Tue Mar 21 15:18:07 2006
Return-path: <pgsql-hackers-owner+M81283@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2LKI7M12970
	for <pgman@candle.pha.pa.us>; Tue, 21 Mar 2006 15:18:07 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id AABA167BBF8;
	Tue, 21 Mar 2006 16:18:04 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 3E6009DC827
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Tue, 21 Mar 2006 16:17:38 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 56406-07
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Tue, 21 Mar 2006 16:17:38 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from stark.xeocode.com (stark.xeocode.com [216.58.44.227])
	by postgresql.org (Postfix) with ESMTP id 21DCF9DC809
	for <pgsql-hackers@postgresql.org>; Tue, 21 Mar 2006 16:17:35 -0400 (AST)
Received: from localhost ([127.0.0.1] helo=stark.xeocode.com)
	by stark.xeocode.com with smtp (Exim 3.36 #1 (Debian))
	id 1FLnI7-0004xB-00; Tue, 21 Mar 2006 15:17:19 -0500
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "Dann Corbit" <DCorbit@connx.com>,
   "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again
References: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com>
	<18732.1142967137@sss.pgh.pa.us>
In-Reply-To: <18732.1142967137@sss.pgh.pa.us>
From: Greg Stark <gsstark@mit.edu>
Organization: The Emacs Conspiracy; member since 1992
Date: 21 Mar 2006 15:17:19 -0500
Message-ID: <87lkv3mu28.fsf@stark.xeocode.com>
Lines: 13
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.128 required=5 tests=[AWL=0.128]
X-Spam-Score: 0.128
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR


Tom Lane <tgl@sss.pgh.pa.us> writes:

> and here are the results using glibc's qsort, which of course isn't
> quicksort at all but some kind of merge sort:
> ...
> Overall: average cratio 0.63 over 525 tests

That looks better both on average and in the worst case. Are the time
constants that much worse that the merge sort still takes longer?

-- 
greg


---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

From pgsql-hackers-owner+M81285@postgresql.org Tue Mar 21 15:38:06 2006
Return-path: <pgsql-hackers-owner+M81285@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2LKc6M14799
	for <pgman@candle.pha.pa.us>; Tue, 21 Mar 2006 15:38:06 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 0917767BBF8;
	Tue, 21 Mar 2006 16:38:03 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 069389DC843
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Tue, 21 Mar 2006 16:37:39 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 60037-07
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Tue, 21 Mar 2006 16:37:39 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id BDC039DC827
	for <pgsql-hackers@postgresql.org>; Tue, 21 Mar 2006 16:37:36 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2LKbZid019858;
	Tue, 21 Mar 2006 15:37:35 -0500 (EST)
To: Greg Stark <gsstark@mit.edu>
cc: "Dann Corbit" <DCorbit@connx.com>,
   "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <87lkv3mu28.fsf@stark.xeocode.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com> <18732.1142967137@sss.pgh.pa.us> <87lkv3mu28.fsf@stark.xeocode.com>
Comments: In-reply-to Greg Stark <gsstark@mit.edu>
	message dated "21 Mar 2006 15:17:19 -0500"
Date: Tue, 21 Mar 2006 15:37:35 -0500
Message-ID: <19857.1142973455@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

Greg Stark <gsstark@mit.edu> writes:
> That looks better both on average and in the worst case. Are the time
> constants that much worse that the merge sort still takes longer?

Keep in mind that this is only counting the number of
comparison-function calls; it's not accounting for any other effects.
In particular, for a large sort operation quicksort might win because of
its more cache-friendly memory access patterns.

The whole question of our qsort vs the system library's qsort probably
needs to be revisited, however, now that we've identified and fixed this
particular performance issue.

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

From pgsql-hackers-owner+M81289@postgresql.org Tue Mar 21 16:27:30 2006
Return-path: <pgsql-hackers-owner+M81289@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2LLRTM20101
	for <pgman@candle.pha.pa.us>; Tue, 21 Mar 2006 16:27:30 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id ACB4F67BBFD;
	Tue, 21 Mar 2006 17:27:27 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 16E0E9DCA0F
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Tue, 21 Mar 2006 17:27:01 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 69903-02
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Tue, 21 Mar 2006 17:27:02 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from stark.xeocode.com (stark.xeocode.com [216.58.44.227])
	by postgresql.org (Postfix) with ESMTP id 107429DC867
	for <pgsql-hackers@postgresql.org>; Tue, 21 Mar 2006 17:26:58 -0400 (AST)
Received: from localhost ([127.0.0.1] helo=stark.xeocode.com)
	by stark.xeocode.com with smtp (Exim 3.36 #1 (Debian))
	id 1FLoNU-0006M4-00; Tue, 21 Mar 2006 16:26:56 -0500
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Greg Stark <gsstark@mit.edu>, "Dann Corbit" <DCorbit@connx.com>,
   "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again
References: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com>
	<18732.1142967137@sss.pgh.pa.us> <87lkv3mu28.fsf@stark.xeocode.com>
	<19857.1142973455@sss.pgh.pa.us>
In-Reply-To: <19857.1142973455@sss.pgh.pa.us>
From: Greg Stark <gsstark@mit.edu>
Organization: The Emacs Conspiracy; member since 1992
Date: 21 Mar 2006 16:26:55 -0500
Message-ID: <87acbjmqu8.fsf@stark.xeocode.com>
Lines: 22
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.128 required=5 tests=[AWL=0.128]
X-Spam-Score: 0.128
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

Tom Lane <tgl@sss.pgh.pa.us> writes:

> Greg Stark <gsstark@mit.edu> writes:
> > That looks better both on average and in the worst case. Are the time
> > constants that much worse that the merge sort still takes longer?
> 
> Keep in mind that this is only counting the number of
> comparison-function calls; it's not accounting for any other effects.
> In particular, for a large sort operation quicksort might win because of
> its more cache-friendly memory access patterns.

My question explicitly recognized that possibility. I'm just a little
skeptical since the comparison function in Postgres is often not some simple
bit of tightly optimized C code, but rather a complex locale sensitive
comparison function or even a bit of SQL expression to evaluate.

Cache effectiveness is may be a minimal factor anyways when the comparison is
executing more than a minimal amount of code. And one extra comparison is
going to cost a lot more too.

-- 
greg


---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not
       match

From pgsql-hackers-owner+M81290@postgresql.org Tue Mar 21 16:48:00 2006
Return-path: <pgsql-hackers-owner+M81290@postgresql.org>
Received: from ams.hub.org (ams.hub.org [200.46.204.13])
	by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id k2LLlxM22215
	for <pgman@candle.pha.pa.us>; Tue, 21 Mar 2006 16:47:59 -0500 (EST)
Received: from postgresql.org (postgresql.org [200.46.204.71])
	by ams.hub.org (Postfix) with ESMTP id 28A9867BBFD;
	Tue, 21 Mar 2006 17:47:57 -0400 (AST)
X-Original-To: pgsql-hackers-postgresql.org@localhost.postgresql.org
Received: from localhost (av.hub.org [200.46.204.144])
	by postgresql.org (Postfix) with ESMTP id 1B4849DCC25
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>; Tue, 21 Mar 2006 17:47:27 -0400 (AST)
Received: from postgresql.org ([200.46.204.71])
	by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024)
	with ESMTP id 72535-05
	for <pgsql-hackers-postgresql.org@localhost.postgresql.org>;
	Tue, 21 Mar 2006 17:47:28 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by postgresql.org (Postfix) with ESMTP id D27239DCC21
	for <pgsql-hackers@postgresql.org>; Tue, 21 Mar 2006 17:47:24 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.13.1/8.13.1) with ESMTP id k2LLlNpJ002194;
	Tue, 21 Mar 2006 16:47:23 -0500 (EST)
To: Greg Stark <gsstark@mit.edu>
cc: "Dann Corbit" <DCorbit@connx.com>,
   "Jonah H. Harris" <jonah.harris@gmail.com>, pgsql-hackers@postgresql.org,
   "Jerry Sievers" <jerry@jerrysievers.com>
Subject: Re: [HACKERS] qsort, once again 
In-Reply-To: <87acbjmqu8.fsf@stark.xeocode.com> 
References: <D425483C2C5C9F49B5B7A41F8944154757D6A1@postal.corporate.connx.com> <18732.1142967137@sss.pgh.pa.us> <87lkv3mu28.fsf@stark.xeocode.com> <19857.1142973455@sss.pgh.pa.us> <87acbjmqu8.fsf@stark.xeocode.com>
Comments: In-reply-to Greg Stark <gsstark@mit.edu>
	message dated "21 Mar 2006 16:26:55 -0500"
Date: Tue, 21 Mar 2006 16:47:23 -0500
Message-ID: <2193.1142977643@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: by amavisd-new at hub.org
X-Spam-Status: No, score=0.113 required=5 tests=[AWL=0.113]
X-Spam-Score: 0.113
X-Mailing-List: pgsql-hackers
List-Archive: <http://archives.postgresql.org/pgsql-hackers>
List-Help: <mailto:majordomo@postgresql.org?body=help>
List-Id: <pgsql-hackers.postgresql.org>
List-Owner: <mailto:pgsql-hackers-owner@postgresql.org>
List-Post: <mailto:pgsql-hackers@postgresql.org>
List-Subscribe: <mailto:majordomo@postgresql.org?body=sub%20pgsql-hackers>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org
Status: OR

Greg Stark <gsstark@mit.edu> writes:
> My question explicitly recognized that possibility. I'm just a little
> skeptical since the comparison function in Postgres is often not some simple
> bit of tightly optimized C code, but rather a complex locale sensitive
> comparison function or even a bit of SQL expression to evaluate.

Yeah, I'd guess the same way, but OTOH at least a few people have
reported that our qsort code is consistently faster than glibc's (and
that was before this fix).  See this thread:
http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php

Currently I believe that we only use our qsort on Solaris, not any other
platform, so if you think that glibc's qsort is better then you've
already got your wish.  It seems to need more investigation though.
In particular, I'm thinking that the various adjustments we've made
to the sort support code over the past month probably invalidate any
previous testing of the point, and that we ought to go back and redo
those comparisons.

			regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings

