Message-ID: <4577288A.8080800@myemma.com>
Date: Wed, 06 Dec 2006 14:31:06 -0600
From: Erik Jones <erik@myemma.com>
User-Agent: Thunderbird 1.5.0.7 (Windows/20060909)
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To: Rafael Martinez <r.m.guerrero@usit.uio.no>
Cc: Ted Allen <tallen@blackducksoftware.com>,
	Stephan Szabo <sszabo@megazone.bigpanda.com>,
	pgsql-performance@postgresql.org
Subject: Re: Problems with an update-from statement and  pg-8.1.4
References: <1165432244.9606.18.camel@bbking.linux>	
	<20061206111630.R8187@megazone.bigpanda.com>	
	<45772022.4050303@blackducksoftware.com>	
	<1165435845.9606.26.camel@bbking.linux> <457725DA.90402@myemma.com>
	<1165436967.9606.32.camel@bbking.linux>
In-Reply-To: <1165436967.9606.32.camel@bbking.linux>
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Rafael Martinez wrote:
> On Wed, 2006-12-06 at 14:19 -0600, Erik Jones wrote:
>   
>> Rafael Martinez wrote:
>>     
>>> On Wed, 2006-12-06 at 14:55 -0500, Ted Allen wrote:
>>>   
>>>       
>>>> Stephan Szabo wrote:
>>>>     
>>>>         
>>>>> On Wed, 6 Dec 2006, Rafael Martinez wrote:
>>>>>       
>>>>>           
>>>>>> mailstats=# EXPLAIN update mail SET spamscore = '-5.026'  FROM mail m,
>>>>>> mail_received mr where mr.mail_id = m.mail_id AND mr.queue_id =
>>>>>> '1GrxLs-0004N9-I1' and mr.mailhost = '129.240.10.47';
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> I don't think this statement does what you expect. You're ending up with
>>>>> two copies of mail in the above one as "mail" and one as "m". You probably
>>>>> want to remove the mail m in FROM and use mail rather than m in the
>>>>> where clause.
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>> Worse yet I think your setting "spamcore" for EVERY row in mail to 
>>>> '-5.026'.  The above solution should fix it though.
>>>>
>>>> -- Ted
>>>>
>>>>     
>>>>         
>>> Thanks for the answers. I think the 'problem' is explain in the
>>> documentation:
>>>
>>> "fromlist
>>>
>>> A list of table expressions, allowing columns from other tables to
>>> appear in the WHERE condition and the update expressions. This is
>>> similar to the list of tables that can be specified in the FROMClause of
>>> a SELECT statement. Note that the target table must not appear in the
>>> fromlist, unless you intend a self-join (in which case it must appear
>>> with an alias in the fromlist)". 
>>>
>>> And as you said, we can not have 'mail m' in the FROM clause. I have
>>> contacted the developers and they will change the statement. I gave then
>>> these 2 examples:
>>>
>>> -------------------------------------------------------------------------------
>>> mailstats=# EXPLAIN update mail SET spamscore = '-5.026'  FROM
>>> mail_received mr where mr.mail_id = mail.mail_id AND mr.queue_id =
>>> '1GrxLs-0004N9-I1' and mr.mailhost = '129.240.10.47';
>>>                                               QUERY PLAN
>>> ------------------------------------------------------------------------------------------------------
>>>  Nested Loop  (cost=0.00..6.54 rows=1 width=57)
>>>    ->  Index Scan using received_queue_id_index on mail_received mr
>>> (cost=0.00..3.20 rows=1 width=4)
>>>          Index Cond: ((queue_id)::text = '1GrxLs-0004N9-I1'::text)
>>>          Filter: (mailhost = '129.240.10.47'::inet)
>>>    ->  Index Scan using mail_pkey on mail  (cost=0.00..3.32 rows=1
>>> width=57)
>>>          Index Cond: ("outer".mail_id = mail.mail_id)
>>> (6 rows)
>>>
>>> mailstats=# explain  update mail SET spamscore = '-5.026' where mail_id
>>> = (select mail_id from mail_received where queue_id = '1GrxLs-0004N9-I1'
>>> and mailhost = '129.240.10.47');
>>>                                              QUERY PLAN
>>> -----------------------------------------------------------------------------------------------------
>>>  Index Scan using mail_pkey on mail  (cost=3.20..6.52 rows=1 width=57)
>>>    Index Cond: (mail_id = $0)
>>>    InitPlan
>>>      ->  Index Scan using received_queue_id_index on mail_received
>>> (cost=0.00..3.20 rows=1 width=4)
>>>            Index Cond: ((queue_id)::text = '1GrxLs-0004N9-I1'::text)
>>>            Filter: (mailhost = '129.240.10.47'::inet)
>>> (6 rows)
>>> -------------------------------------------------------------------------------
>>>   
>>>       
>> Look again at the estimated costs of those two query plans.  You haven't 
>> gained anything there.  Try this out:
>>
>> EXPLAIN UPDATE mail
>> SET spamscore = '-5.026'
>> FROM mail_received mr
>> WHERE mail.mail_id = mr.mail_id AND mr.queue_id = '1GrxLs-0004N9-I1' ;
>>
>>     
>
> Haven't we? 
>
> * In the statement with problems we got this:
> Nested Loop  (cost=0.00..932360.78 rows=7184312 width=57)
>
> * In the ones I sent:
> Nested Loop  (cost=0.00..6.54 rows=1 width=57)
> Index Scan using mail_pkey on mail  (cost=3.20..6.52 rows=1 width=57)
>
> * And in the last one you sent me:
> ------------------------------------------------------  
> Nested Loop  (cost=0.00..6.53 rows=1 width=57)
>    ->  Index Scan using received_queue_id_index on mail_received mr
> (cost=0.00..3.20 rows=1 width=4)
>          Index Cond: ((queue_id)::text = '1GrxLs-0004N9-I1'::text)
>    ->  Index Scan using mail_pkey on mail  (cost=0.00..3.32 rows=1
> width=57)
>          Index Cond: (mail.mail_id = "outer".mail_id)
> (5 rows)
> ------------------------------------------------------
>
> I can not see the different.
>
> regards,
>   
Ah, sorry, I was just looking at the two that you sent in your last 
message thinking that they were 'old' and 'new',  not both 'new'.  My bad...

-- 
erik jones <erik@myemma.com>
software development
emma(r)