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From: Brian Fehrle <brianf@consistentstate.com>
To: pgsql-performance@postgresql.org
Subject: Re: Trying to eliminate union and sort
Date: Mon, 15 Jul 2013 10:12:20 -0600
Message-ID: <51E41F64.7090102@consistentstate.com> (raw)
In-Reply-To: <51E086A1.2080504@agliodbs.com>
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On 07/12/2013 04:43 PM, Josh Berkus wrote:
>> As for the counts on the tables:
>> table1      3,653,472
>> table2      2,191,314
>> table3    25,676,589
>>
>> I think it's safe to assume right now that any resulting joins are not
>> one-to-one
> Hmmm?  How is doing a subselect in the SELECT clause even working, then?
>
Oh my, this is sad. the query in all returns 9,955,729 rows, so the sub 
queries are run on each of these resulting rows, however in this entire 
result set, subquery 1 returns 16 distinct rows, subquery 2 returns 63 
different rows, but those sub queries are run over 10 million times to 
return these few distinct rows. So it's running many times, but 
returning the same small set of data over and over again.

- Brian F




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