Re: Re-Reason of Slowness of Query

Vitalii Tymchyshyn <tivv00@xxxxxxxxx> · Wed, 23 Mar 2011 12:37:58 +0200



    23.03.11 12:19, Adarsh Sharma написав(ла):
    
      
      Vitalii Tymchyshyn wrote:
      
        
        23.03.11 12:10, Adarsh Sharma написав(ла):
        
          
          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