Buffer cache statistics (ratios) and CBO SQL optimization?

Can some guru please enlighten me?

I have been reading the threads about why buffer hit ratios are useless and yet I'm having trouble reconciling this with the Oracle documentation and my limited understanding of Oracle tuning.

• Why does the CBO want to know estimates about the index buffer hit ratio (optimizer_index_caching parameter)?
• Why did Oracle offer the KEEP pool and the other blocksize buffer pools?
• Why do Oracle experts place such high emphasis on "physical" block reads when optimizing SQL?

For example, Vadim Tropashko, who works for Real World Performance group at Oracle says caching ratios are an important part of CBO internals because the CBO's goal is to minimize "physical" reads.

http://www.dbazine.com/tropashko3.html

3) Data Caching. With caching a simplistic model where the cost is based upon the number of logical IOs is no longer valid: cost model adjustment and caching statistics is necessary. Received on Wed Dec 24 2003 - 09:38:47 CST