Hi, Zardosht!
On Oct 07, Zardosht Kasheff wrote:
> Hello,
>
> I would like to make sure that I understand the meaning of the
> functions handler::scan_time and handler::read_time. Both are to be
> estimates of the cost of doing some operations on a table.
>
> handler::scan_time estimates the cost of doing a full table scan.
> handler::read_time estimates the cost of reading some number of rows
> from some number of ranges in an index (assumed to be a non-covering
> index).
>
> The estimates requested are meant to be disk seeks.
>
> I have gathered this from reading comments in different handler implementations.
>
> The questions that I need help on are the following:
> 1) Is my understanding above accurate?
Yes. At least it matches mine :)
> 2) Does the absolute value returned affect the query plan, or is it
> the relative value between these two functions?
I'm afraid that absolute values matter. And the the units of
scan_time/read_time are quite arbitrary and there's no normalization
procedure that would help to select them correctly :(
I suppose that other engines simply tweak these numbers until they get
satisfactory query plans.
Regards / Mit vielen Grüßen,
Sergei
--
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/ |/ /_ __/ __/ __ \/ / Sergei Golubchik <serg@stripped>
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