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Update TODO.detail/qsort.
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Bruce Momjian
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@ -582,3 +582,409 @@ broadcast)---------------------------
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TIP 2: Don't 'kill -9' the postmaster
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From kleptog@svana.org Mon Dec 19 06:37:51 2005
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Date: Mon, 19 Dec 2005 12:37:30 +0100
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From: Martijn van Oosterhout <kleptog@svana.org>
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To: Dann Corbit <DCorbit@connx.com>
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cc: Tom Lane <tgl@sss.pgh.pa.us>, Qingqing Zhou <zhouqq@cs.toronto.edu>,
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Bruce Momjian <pgman@candle.pha.pa.us>,
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Luke Lonergan <llonergan@greenplum.com>, Neil Conway <neilc@samurai.com>,
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pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] Re: Which qsort is used
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Message-ID: <20051219113724.GD12251@svana.org>
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Reply-To: Martijn van Oosterhout <kleptog@svana.org>
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References: <D425483C2C5C9F49B5B7A41F8944154757D38D@postal.corporate.connx.com>
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On Fri, Dec 16, 2005 at 10:43:58PM -0800, Dann Corbit wrote:
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> I am actually quite impressed with the excellence of Bentley's sort out
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> of the box. It's definitely the best library implementation of a sort I
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> have seen.
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I'm not sure whether we have a conclusion here, but I do have one
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question: is there a significant difference in the number of times the
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comparison routines are called? Comparisons in PostgreSQL are fairly
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expensive given the fmgr overhead and when comparing tuples it's even
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worse.
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We don't want to accedently pick a routine that saves data shuffling by
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adding extra comparisons. The stats at [1] don't say. They try to
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factor in CPU cost but they seem to use unrealistically small values. I
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would think a number around 50 (or higher) would be more
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representative.
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[1] http://www.cs.toronto.edu/~zhouqq/postgresql/sort/sort.html
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Have a nice day,
|
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--=20
|
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Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
|
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> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
|
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> tool for doing 5% of the work and then sitting around waiting for someone
|
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> else to do the other 95% so you can sue them.
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From: Manfred Koizar <mkoi-pg@aon.at>
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To: Tom Lane <tgl@sss.pgh.pa.us>
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cc: "Dann Corbit" <DCorbit@connx.com>, "Qingqing Zhou" <zhouqq@cs.toronto.edu>,
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"Neil Conway" <neilc@samurai.com>, pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] Re: Which qsort is used
|
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Date: Thu, 22 Dec 2005 01:43:34 +0100
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Message-ID: <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com>
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References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us>
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On Sat, 17 Dec 2005 00:03:25 -0500, Tom Lane <tgl@sss.pgh.pa.us>
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wrote:
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>I've still got a problem with these checks; I think they are a net
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>waste of cycles on average. [...]
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> and when they fail, those cycles are entirely wasted;
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>you have not advanced the state of the sort at all.
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How can we make the initial check "adavance the state of the sort"?
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One answer might be to exclude the sorted sequence at the start of the
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array from the qsort, and merge the two sorted lists as the final
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stage of the sort.
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Qsorting N elements costs O(N*lnN), so excluding H elements from the
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sort reduces the cost by at least O(H*lnN). The merge step costs O(N)
|
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plus some (<=50%) more memory, unless someone knows a fast in-place
|
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merge. So depending on the constant factors involved there might be a
|
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usable solution.
|
||||
|
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I've been playing with some numbers and assuming the constant factors
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to be equal for all the O()'s this method starts to pay off at
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H for N
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20 100
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130 1000
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8000 100000
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Servus
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Manfred
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From pgsql-hackers-owner+M77795=pgman=candle.pha.pa.us@postgresql.org Thu Dec 22 02:02:28 2005
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Date: Thu, 22 Dec 2005 08:01:00 +0100
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From: Martijn van Oosterhout <kleptog@svana.org>
|
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To: Manfred Koizar <mkoi-pg@aon.at>
|
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cc: Tom Lane <tgl@sss.pgh.pa.us>, Dann Corbit <DCorbit@connx.com>,
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Qingqing Zhou <zhouqq@cs.toronto.edu>,
|
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Bruce Momjian <pgman@candle.pha.pa.us>,
|
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Luke Lonergan <llonergan@greenplum.com>, Neil Conway <neilc@samurai.com>,
|
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pgsql-hackers@postgresql.org
|
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Subject: Re: [HACKERS] Re: Which qsort is used
|
||||
Message-ID: <20051222070057.GA21783@svana.org>
|
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Reply-To: Martijn van Oosterhout <kleptog@svana.org>
|
||||
References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us> <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com>
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On Thu, Dec 22, 2005 at 01:43:34AM +0100, Manfred Koizar wrote:
|
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> Qsorting N elements costs O(N*lnN), so excluding H elements from the
|
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> sort reduces the cost by at least O(H*lnN). The merge step costs O(N)
|
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> plus some (<=3D50%) more memory, unless someone knows a fast in-place
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> merge. So depending on the constant factors involved there might be a
|
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> usable solution.
|
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|
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But where are you including the cost to check how many cells are
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already sorted? That would be O(H), right? This is where we come back
|
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to the issue that comparisons in PostgreSQL are expensive. The cpu_cost
|
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in the tests I saw so far is unrealistically low.
|
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|
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> I've been playing with some numbers and assuming the constant factors
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> to be equal for all the O()'s this method starts to pay off at
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> H for N
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> 20 100 20%
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> 130 1000 13%
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> 8000 100000 8%
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Hmm, what are the chances you have 100000 unordered items to sort and
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that the first 8% will already be in order. ISTM that that probability
|
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will be close enough to zero to not matter...
|
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Have a nice day,
|
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--=20
|
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Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
|
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> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
|
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> tool for doing 5% of the work and then sitting around waiting for someone
|
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> else to do the other 95% so you can sue them.
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To: Martijn van Oosterhout <kleptog@svana.org>
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cc: Tom Lane <tgl@sss.pgh.pa.us>, Dann Corbit <DCorbit@connx.com>,
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Subject: Re: [HACKERS] Re: Which qsort is used
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Date: Thu, 22 Dec 2005 22:58:31 +0100
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References: <D425483C2C5C9F49B5B7A41F8944154757D386@postal.corporate.connx.com> <3148.1134795805@sss.pgh.pa.us> <odqjq1tv6cb77ri4df0aehqal8o0ljtkar@4ax.com> <20051222070057.GA21783@svana.org>
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On Thu, 22 Dec 2005 08:01:00 +0100, Martijn van Oosterhout
|
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<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
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user