Thread: abou the cost estimation

Hello, All   I have read the source code  /cvsroot/pgsql/src/backend/optimizer/path/costsize.c and there is a function
cost_sort(...).I think the code in 464 to 465 lines must be changed to:    startup_cost += npageaccesses *   (1.0 +
cost_nonsequential_access(1))* 0.5;
 

The original code is:    startup_cost += npageaccesses *    (1.0 + cost_nonsequential_access(npages)) * 0.5;
Can any one discuss about this issue with me ? Thanks for your response very much!
----------------------Guo long jiang. 2002-9-6 
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ljguo_1234 <ljguo_1234@sina.com> writes:
>     I have read the source code  /cvsroot/pgsql/src/backend/optimizer/path/costsize.c and there is a function
cost_sort(...).I think the code in 464 to 465 lines must be changed to:
 
>      startup_cost += npageaccesses *
>        (1.0 + cost_nonsequential_access(1)) * 0.5;

That would be wrong.  Note the definition of cost_nonsequential_access:
*      Estimate the cost of accessing one page at random from a relation*      (or sort temp file) of the given size in
pages.
        regards, tom lane