Hi Serg
On 25/05/2009, at 10:41 PM, Sergei Golubchik wrote:
> On May 25, Arjen Lentz wrote:
>>> On May 25, Arjen Lentz wrote:
>>>> I mean, if I do a query about 1 table on an information schema
>>>> table, then mysqld will first run through all tables in all
>>>> databases, prouduce the temporary table of everything (most likely
>>>> on disk), and then run my query on it.
>>>
>>> Depends on the query. If you do a query about 1 table on an
>>> information schema table, like, for example, SELECT ... FROM
>>> I_S.TABLES WHERE SCHEMA_NAME='d1' AND TABLE_NAME='t1' - MySQL will
>>> not run through all tables in all databases. It'll open d1/t1.frm
>>> directly. Or may not do even that, for example for SELECT TABLE_NAME
>>> FROM ... - it doesn't need to open frm, it's enough to verify that
>>> the frm file exists.
>>
>> Cognitive dissonance for me there.
>>
>> Is your statement above based on looking at the source code, or a
>> presumption of how it works?
>
> It's based on remembering how it used to work, BUG# and WL# with the
> task of implementing the above optimization, reviewing the patch, and
> looking through the source code briefly few days ago.
>
> Oh, yes, and presumption that I developed after all the above :)
>
>> If the latter, can you please take a peek at the actual code and
>> confirm? Thanks
>
> BUG#19588
> WL#3732
> http://dev.mysql.com/doc/refman/5.1/en/information-schema-optimization.html
Ah, 5.1 foo (the page does not exist for 5.0)
That explains.
Thanks
Cheers,
Arjen.
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