Hi Jeff Thank you very much.
>I determined this by changing each cost parameter and running explain, >to see how much each one changed the cost estimate (after verifying >the overall plan did not change). your method is so smart! Jian Gao 2012/11/9 Jeff Janes <jeff.ja...@gmail.com> > On Wed, Nov 7, 2012 at 11:17 PM, 高健 <luckyjack...@gmail.com> wrote: > > Hi all: > > > > > > > > I want to see the explain plan for a simple query. My question is : > How > > is the cost calculated? > > > > > > > > The cost parameter is: > > > > > > > > random_page_cost = 4 > > > > seq_page_cost = 1 > > > > cpu_tuple_cost =0.01 > > > > cpu_operator_cost =0.0025 > > The cost is estimates as 2*random_page_cost + cpu_tuple_cost + > cpu_index_tuple_cost + 100* cpu_operator_cost. > > I determined this by changing each cost parameter and running explain, > to see how much each one changed the cost estimate (after verifying > the overall plan did not change). > > I was surprised the multiplier for cpu_operator_cost was that high. > > The two random_page_costs are one for the index leaf page and one for > the table page. Higher pages in the index are assumed to be cached > and thus not charged for IO. > > ... > > > Firstly, database need to search for 9 index pages by sequential to > find > > the index entry. For each index page in memory, every “index tuple” > need > > to be scanned. > > That is not how indexes are traversed. > > Cheers, > > Jeff >
