Message-ID: <48340687.9010501@mark.mielke.cc>
Date: Wed, 21 May 2008 07:24:55 -0400
From: Mark Mielke <mark@mark.mielke.cc>
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To: Albert Cervera Areny <albert@sedifa.com>
CC: Richard Huxton <dev@archonet.com>, pgsql-performance@postgresql.org
Subject: Re: Posible planner improvement?
References: <200805211228.48248.albert@sedifa.com>
	<4833FDE5.8090603@archonet.com>
	<200805211311.25016.albert@sedifa.com>
In-Reply-To: <200805211311.25016.albert@sedifa.com>
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A Dimecres 21 Maig 2008, Richard Huxton va escriure:
>> Albert Cervera Areny wrote:
>>     
>>> I've got a query similar to this:
>>>
>>> select * from t1, t2 where t1.id > 158507 and t1.id = t2.id;
>>>
>>> That took > 84 minutes (the query was a bit longer but this is the part
>>> that made the difference) after a little change the query took ~1 second:
>>>
>>> select * from t1, t2 where t1.id > 158507 and t2.id > 158507 and t1.id =
>>> t2.id;
>>>       
>> Try posting EXPLAIN ANALYSE SELECT ... for both of those queries and
>> we'll see why it's better at the second one.
>>     

Even if the estimates were off (they look a bit off for the first 
table), the above two queries are logically identical, and I would 
expect the planner to make the same decision for both.

I am curious - what is the result of:

    select * from t1, t2 where t2.id > 158507 and t1.id = t2.id;

Is it the same speed as the first or second, or is a third speed entirely?

If t1.id = t2.id, I would expect the planner to substitute them freely 
in terms of identities?

Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>