From: Michael Dykman
Date: September 9 2010 7:16pm
Subject: Re: Trying to remove a filesort.
List-Archive: http://lists.mysql.com/mysql/222906
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How many rows before the GROUP BY?  Group by is, in effect a sorting
process..  perhaps that contains enough data to justify going to disk.

What is the value of the variable sort_buffer_size?
         show variables like '%sort%';

 - md

On Thu, Sep 9, 2010 at 3:04 PM, Phil <freedc.bok@stripped> wrote:
> On average it would be between 10 and 40, certainly no more than 100.
>
>
> On Thu, Sep 9, 2010 at 2:57 PM, Michael Dykman <mdykman@stripped> wrote:
>>
>> The filesort is probably necessary because of the number of rows in
>> the result set to be ordered. =A0How many rows do you get out of this
>> query?
>>
>> =A0- michael dykman
>>
>> On Thu, Sep 9, 2010 at 1:53 PM, Phil <freedc.bok@stripped> wrote:
>> > I wonder if anyone could help with a query which I've been unable to
>> > prevent
>> > from using a filesort. Might be something obvious I'm overlooking!
>> >
>> > I have a table which tracks milestones in distributed computing projec=
ts
>> >
>> > Create Table: CREATE TABLE `boinc_milestone` (
>> > =A0`proj` char(6) NOT NULL,
>> > =A0`id` int(11) NOT NULL,
>> > =A0`stat_date` date NOT NULL DEFAULT '0000-00-00',
>> > =A0`milestone_type` char(1) NOT NULL DEFAULT '0',
>> > =A0`milestone` double NOT NULL DEFAULT '0',
>> > =A0`cpid` varchar(32) DEFAULT NULL,
>> > =A0`team` int(11) DEFAULT NULL,
>> > =A0PRIMARY KEY (`proj`,`id`,`stat_date`,`milestone`),
>> > =A0KEY `two` (`proj`,`stat_date`,`id`,`milestone`),
>> > =A0KEY `cpid` (`cpid`,`proj`,`id`,`stat_date`,`milestone`),
>> > =A0KEY `team` (`proj`,`team`,`id`,`stat_date`,`milestone`)
>> > ) ENGINE=3DMyISAM DEFAULT CHARSET=3Dlatin1
>> >
>> > These are added to on a daily basis as users pass the various credit
>> > milestones so for instance you can end up with rows for
>> > 1000,5000,10000,50000,1000000 etc on different dates as time goes on.
>> >
>> > Now on one page for display I want to show the latest milestone for ea=
ch
>> > project for a particular cpid. The query I use is as follows:
>> >
>> > select a.proj,a.id,max(stat_date),max(a.milestone) as
>> > milestone,b.description
>> > =A0 =A0 =A0 =A0 from boinc_milestone a join boinc_projects b on a.proj=
 =3D b.proj
>> > =A0 =A0 =A0 =A0 where cpid =3D '$cpid'
>> > =A0 =A0 =A0 =A0 group by proj
>> > =A0 =A0 =A0 =A0 order by stat_date desc
>> >
>> > The order by causes the filesort and I can't find an easy way around i=
t.
>> >
>> > mysql> explain =A0select a.proj,a.id,max(stat_date),max(a.milestone) a=
s
>> > milestone,b.description from boinc_milestone a join boinc_projects b o=
n
>> > a.proj =3D b.proj where cpid =3D 'XXX' group by proj order by stat_dat=
e\G
>> > *************************** 1. row ***************************
>> > =A0 =A0 =A0 =A0 =A0 id: 1
>> > =A0select_type: SIMPLE
>> > =A0 =A0 =A0 =A0table: a
>> > =A0 =A0 =A0 =A0 type: ref
>> > possible_keys: PRIMARY,two,cpid,team
>> > =A0 =A0 =A0 =A0 =A0key: cpid
>> > =A0 =A0 =A0key_len: 35
>> > =A0 =A0 =A0 =A0 =A0ref: const
>> > =A0 =A0 =A0 =A0 rows: 1
>> > =A0 =A0 =A0 =A0Extra: Using where; Using index; Using temporary; Using=
 filesort
>> > *************************** 2. row ***************************
>> > =A0 =A0 =A0 =A0 =A0 id: 1
>> > =A0select_type: SIMPLE
>> > =A0 =A0 =A0 =A0table: b
>> > =A0 =A0 =A0 =A0 type: eq_ref
>> > possible_keys: PRIMARY
>> > =A0 =A0 =A0 =A0 =A0key: PRIMARY
>> > =A0 =A0 =A0key_len: 10
>> > =A0 =A0 =A0 =A0 =A0ref: stats.a.proj
>> > =A0 =A0 =A0 =A0 rows: 1
>> > =A0 =A0 =A0 =A0Extra: Using where
>> > 2 rows in set (0.00 sec)
>> >
>> > I could just remove the order by altogether and perform the sort in ph=
p
>> > afterwards I guess but any other ideas?
>> >
>> > Thanks
>> >
>> > Phil
>> >
>> > --
>> > Distributed Computing stats
>> > http://stats.free-dc.org
>> >
>>
>>
>>
>> --
>> =A0- michael dykman
>> =A0- mdykman@stripped
>>
>> =A0May the Source be with you.
>
>
>
> --
> Distributed Computing stats
> http://stats.free-dc.org
>



--=20
=A0- michael dykman
=A0- mdykman@stripped

=A0May the Source be with you.