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.janes@gmail.com>On Wed, Nov 7, 2012 at 11:17 PM, 高健 <
luckyjackgao@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
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
