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From:Zardosht Kasheff Date:October 7 2009 8:34pm
Subject:Re: understanding handler::read_time and handler::scan_time
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I realize this may be difficult to answer, but any examples on when
absolute values matter?

Thanks
-Zardosht

On Wed, Oct 7, 2009 at 4:27 PM, Sergei Golubchik <serg@stripped> wrote:
> 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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Thread
understanding handler::read_time and handler::scan_timeZardosht Kasheff7 Oct
  • Re: understanding handler::read_time and handler::scan_timeSergei Golubchik7 Oct
    • Re: understanding handler::read_time and handler::scan_timeZardosht Kasheff7 Oct
      • Re: understanding handler::read_time and handler::scan_timeZardosht Kasheff8 Oct
        • Re: understanding handler::read_time and handler::scan_timeMichael Widenius8 Oct
        • Re: understanding handler::read_time and handler::scan_timeSergei Golubchik8 Oct
          • Re: understanding handler::read_time and handler::scan_timeZardosht Kasheff8 Oct
            • Re: understanding handler::read_time and handler::scan_timeSergei Golubchik8 Oct
      • Re: understanding handler::read_time and handler::scan_timeSergei Golubchik8 Oct
      • Re: understanding handler::read_time and handler::scan_timeMichael Widenius8 Oct