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From:Evgeny Potemkin Date:October 9 2009 12:01pm
Subject:Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384
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Hi,

Answers are inline.

Regards, Evgen.

Sergei Golubchik wrote:
> Hi, Evgeny!
>
> On Oct 08, Evgeny Potemkin wrote:
>>   3147 Evgeny Potemkin	2009-10-08
>>        Bug#34384: Slow down on constant conversion.
>>        When values of different types are compared they're converted to
>>        a type that allows correct comparison. This conversion is done
>>        for each comparison and takes some time. When a constant is
>>        being compared it's possible to cache the value after conversion
>>        to speedup comparison.
>
> On IRC you told me that "SELECT caches the converted value, while UPDATE
> does not". Where/how SELECT was doing it ? Why UPDATE wasn't ?
I was wrong, constants are cached only if they can be compared as ints by 
convert_constant_item. It's called at fix_fields, thus it's available for UPDATE 
also.

>
> Why do you need to introduce the concept of "late caching" ?
Because the value to be cached isn't always available at the moment of cache 
creation.
Here is an example from our test suite:
SELECT (SELECT 1.5,'c','a') = ROW(1.5,2,'a');
Subselect uses Item_cache to cache it's result and it has the type of the column.
Comparators for row comparison are created at the fix_field moment and as 
subselect is only prepared subselect's cache doesn't contain anything.
Because of this here we will have wrong result.

>
> Regards / Mit vielen Grüßen,
> Sergei
>
Thread
bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384Evgeny Potemkin8 Oct
  • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147)Bug#34384Sergei Golubchik8 Oct
    • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384Evgeny Potemkin9 Oct
      • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384Roy Lyseng9 Oct
        • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384Evgeny Potemkin13 Oct
  • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147) Bug#34384Øystein Grøvlen9 Oct
  • Re: bzr commit into mysql-5.1-bugteam branch (epotemkin:3147)Bug#34384Sergei Golubchik14 Oct