Re: Cache a table

From: Mladen Gogala <mladen_at_wangtrading.com>
Date: Tue, 21 Oct 2003 13:14:28 -0800
Message-ID: <F001.005D3E46.20031021131428@fatcity.com>

Did anyone try to benchmark the touch count based algorithm against the old LRU list? LRU list had advantage of being intuitive, while touch count algorithm is depending on many parameters for which I don't exactly understand the impact. LRU list parameters were essentially defining the desired length of the free buffers list, while the touch count parameters are all undocumented and are signifying the size of touch pool, the interval in which block has to be touched if the touch count is to increase, the required touch count to be moved to the hot pool and alike. Is it more efficient then the previous easy and understandable LRU lists or not? Touch counts are visible as TCH in X$BH. I still have no clue what "TIM" is.

On 10/21/2003 04:39:33 PM, Wolfgang Breitling wrote:

> Before Oracle 8 and the new touch count algorithm the cache attribute  
> made sense. If a small, frequently used table was read by a full  
> scan, it would have been put at the end of the LRU chain eligible to  
> be aged out immediately, quite possibly by itself if it consisted of  
> more than ~ db_file_multiblock_read_count blocks, i.e. the 2nd or 3rd  
> full scan read would already override the previously read blocks.  
> Marking the table as CACHEd prevented that.
> 
> At 01:09 PM 10/21/2003, you wrote:

>> I always wondered why Oracle thought this was a useful table
>> attribute.
>>
>> My gut feeling is that it is an extra that does little.
>>
>> For example, say we want to keep a code table in memory because it
>> is
>> constantly being hit for column verifiction. By definition, if a
>> table is
>> constantly being queried, it's segments will be in memory because
>> they never
>> age out. That sounds like cacheing to me.
>>
>> And then I remember a specific piece of Oracle documentation saying
>> that,
>> even though we may mark a table to be "cached", it *still* may be
>> aged out
>> if memory is needed for other data blocks.
>>
>> Like I said, sounds a little like "here you have it, and here you
>> don't".
>>
>> I'm sure that my impression is wrong and someone will correct me.
>> But I
>> doubt I will use the "CACHE" option anytime soon.
>>
>> Tom Mercadante
>> Oracle Certified Professional

> 
> Wolfgang Breitling
> Oracle7, 8, 8i, 9i OCP DBA
> Centrex Consulting Corporation
> http://www.centrexcc.com
> 
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Mladen Gogala
Oracle DBA

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Author: Mladen Gogala
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Received on Tue Oct 21 2003 - 16:14:28 CDT