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Home -> Community -> Mailing Lists -> Oracle-L -> RE: how to set optimizer_index_cost_adj and cache parameters?
"The default cost of using the bad index is five because Oracle expects to visit one index leaf block and one table block to pick up the relevant data. 1 + 4 = 5."
You probably meant "four table blocks"
Waleed
-----Original Message-----
From: Jonathan Lewis [mailto:jonathan_at_jlcomp.demon.co.uk]
Sent: Friday, March 26, 2004 1:22 PM
To: oracle-l_at_freelists.org
Subject: Re: how to set optimizer_index_cost_adj and cache parameters?
Waleed
Thanks for the note. You're absolutely right,
I sent in a note to the editor just before publication because I had missed out the phrase "of the clustering factor" completely; unfortunately my explanation of where to put it wasn't quite clear enough.
I'll send her a correction a.s.a.p
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
April 2004 Iceland http://www.index.is/oracleday.php June 2004 UK - Optimising Oracle Seminar
Hi Jonathan,
This is from the article:
"Essentially, this formula tells Oracle the number of block visits (see side
bar) that a process will have to make to collect all the data from the
table. If the selectivity (fraction of the table's data) is X, then it seems
reasonable that the number of index leaf blocks to visit will be X% of the
clustering_factor; and the number of table block visits will also be X%. Add
on the fact that you have to walk from the root to the leaf level just once,
and you have the formula."
I believe it's the other way around:
the number of index leaf blocks = X% of leaf blocks the number of table block visits = X% of clustering_factor
Regards,
Waleed
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