Re: Is there something wrong with my test case?

Fabio Pardi <f.pardi@xxxxxxxxxxxx> · Mon, 7 Jan 2019 11:53:40 +0100



    Hi Thiemo, 

    
    On 07/01/2019 11:30, Thiemo Kellner
      wrote:

    
      Hi HP
      

      Thanks for your reply.
      

      Quoting "Peter J. Holzer" <hjp-pgsql@xxxxxx>:
      

      On 2018-12-25 11:54:11 +0000, Thiemo
        Kellner wrote:
        

        [three different but functionally equivalent queries]
        

        Explain analyze verbose showed for:
          

          A (cost=264.72..626.97 rows=31 width=90) (actual
          time=1.117..1.117 rows=0
          

          loops=1)
          

          C (cost=0.42..611.19 rows=31 width=52) (actual
          time=2.217..2.217 rows=0
          

          loops=1)
          

        626.97 doesn't seem "much higher" to me than 611.19. I would
        call that
        

        "about the same".
        

      So would I but the cost is given as a range. Taking the the
      average somewhat 400 compare to somewhat 300. I do not know
      whether averaging is appropriate here.
      

    The cost is not a range. The 2 numbers you see are:

    
        Estimated start-up cost. This is the time expended before the
          output phase can begin, e.g., time to do the sorting in a sort
          node.
      
      Estimated total cost. This is stated on the assumption that
        the
        plan node is run to completion, i.e., all available rows are
        retrieved. In practice a node's parent node might stop short of
        reading all available rows (see the LIMIT
        example below).

      
    As you can read here: 

    
    https://www.postgresql.org/docs/current/using-explain.html

    
      regards,
    fabio pardi