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Re: Re-Reason of Slowness of Query

From: Vitalii Tymchyshyn <tivv00(at)gmail(dot)com>
To: Adarsh Sharma <adarsh(dot)sharma(at)orkash(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Re-Reason of Slowness of Query
Date: 2011-03-23 10:37:58
Message-ID: 4D89CD86.4000008@gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
23.03.11 12:19, Adarsh Sharma ???????(??):
> Vitalii Tymchyshyn wrote:
>> 23.03.11 12:10, Adarsh Sharma ???????(??):
>>> I just want to retrieve that id 's from page_content which do not 
>>> have any entry in clause2 table.
>>>
>> Then
>> select distinct(p.crawled_page_id) from page_content p
>>  where NOT EXISTS (select 1 from  clause2 c where c.source_id = 
>> p.crawled_page_id);
>> is correct query.
>>
>
> I can't understand how*select 1 from  clause2 c where c.source_id = 
> p.crawled_page_id works too, *i get my output .
>
> What is the significance of 1 here.
No significance. You can put anything there. E.g. "*". Simply arbitrary 
constant. Exists checks if there were any rows, it does not matter which 
columns are there or what is in this columns.

Best regards, Vitalii Tymchyshyn

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