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List:       mysql
Subject:    Re: Re: RE: performance tunning 4.x
From:       Victoria Reznichenko <victoria.reznichenko () ensita ! net>
Date:       2002-08-31 10:48:00
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mos,
Friday, August 30, 2002, 7:04:25 PM, you wrote:

m> At 07:58 AM 8/30/2002, you wrote:
>>mos,
>>Thursday, August 29, 2002, 5:45:05 PM, you wrote:
>>
>> >>Start by looking at the MySQL documentation:
>> >>http://www.mysql.com/doc/en/Query_Cache.html
>> >>Read the entire 6.9 section.
>> >>
>> >>It offers quite a bit of information on how the cache operates, how
>> >>you should configure it and so on.
>>
>>m>          I read it too but I have a question about the cache, this sentence
>>m> in particular.
>>
>>m>  NOTE: The query cache does not return stale data. When data is modified,
>>m> any relevant entries in the query cache are flushed.
>>
>>m> If I have a select statement like:
>>
>>m> select col1, col2 from table1 where col1='A'
>>
>>[skip]
>>
>>m> c) Or will any update to the table automatically flush the cache?

m> Victoria,

>>Yes, relevant entries in the query cache are flushed when any changes were 
>>made on the
>>table.

m> Can you define "relevant"? Do you mean all caches for that table?

m> Are you saying any change at all to the table, say I add a row, delete a 
m> row, or modify a row even if it is not in the scope of the cached query, 
m> will cause all caches for that table to be discarded? Argghhh! :-O

Yes.
And it's described in the manual:
    http://www.mysql.com/doc/en/Query_Cache_How.html


m> Example:

m> select customer_name from customers where state = 'NY'  //<-Adds query to cache

m> insert into customers (customer_name, state) values('jones', 
m> 'CA')  //<-Flushes the cache for customers table?

m> select customer_name from customers where state = 'NY'  //<-No longer in cache?

m> If you are correct, then I feel caching is useful primarily for readonly 
m> access to a table. As soon as a row is modified in any way, the caches for 
m> that table are flushed. Would you agree?

Yes.




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