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Re: Posible planner improvement?

From: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
To: Albert Cervera Areny <albert(at)sedifa(dot)com>
Cc: Richard Huxton <dev(at)archonet(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Posible planner improvement?
Date: 2008-05-21 11:24:55
Message-ID: 48340687.9010501@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)mielke(dot)cc>


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