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From:Zardosht Kasheff Date:October 8 2009 3:36am
Subject:Re: understanding handler::read_time and handler::scan_time
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Another question,

are we guaranteed that the return value of scan_time will not cause
the optimizer to use a full table scan over a covering index over a
portion of the table?

Thanks
-Zardosht

On Wed, Oct 7, 2009 at 4:34 PM, Zardosht Kasheff <zardosht@stripped> wrote:
> 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
>>
>> --
>>   __  ___     ___ ____  __
>>  /  |/  /_ __/ __/ __ \/ /   Sergei Golubchik
> <serg@stripped>
>>  / /|_/ / // /\ \/ /_/ / /__  Principal Software Engineer/Server
> Architect
>> /_/  /_/\_, /___/\___\_\___/  Sun Microsystems GmbH, HRB München
> 161028
>>       <___/              
>    Sonnenallee 1, 85551 Kirchheim-Heimstetten
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>> Vorsitzender des Aufsichtsrates: Martin Häring
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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