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Oracle Metric Total Index Scans Per Sec

Oracle Consulting Tips by Burleson


The Total Index Scans Per Sec Oracle metric is the total number of index scans per second.

Is it possible to query the Oracle environment and intelligently determine the optimal setting for optimizer_index_cost_adj ?  The optimizer_index_cost_adj parameters default to a value of 100 and can range in value from 1 to 10,000.  A value of 100 means that equal weight is given to index versus multi-block reads.  In other words, optimizer_index_cost_adj can be thought of as a “how much do I like full-table scans?” parameter. 

With a value of 100, the CBO likes full-table scans and index scans equally, and a number lower than 100 tells the CBO that index scans are faster than full-table scans.  However, even with a super-low setting (optimizer_index_cost_adj  =1), the CBO will still choose full-table scans against no-brainers, like tiny tables that reside on two blocks.

In sum, the optimizer_index_cost_adj  parameter is a weight that can be applied to the relative cost of physical disk reads for two types of block access:

§       A single-block read (i.e. index fetch by ROWID)

§       A multi-block read (i.e. a full-table scan, OPQ, sorting)

Physical disk speed is an important factor in weighing these costs.  As disk access speed increases, the costs of a full-table scan versus single block reads can become negligible.  For example, the new TMS RamSan-210 solid-state disk provides up to 100,000 I/Os per second, six times faster than traditional disk devices.  In a solid-state disk environment, disk I/O is much faster, and multi-block reads are far cheaper than traditional disks.

If you are looking to enhance your Oracle tuning skills, you may enjoy Rampant TechPress – Oracle Tuning: The Definitive Reference by Donald K. Burleson , with over 900 pages of BC's favorite tuning tips & scripts. 

 You can buy it directly from the publisher and get instant access to the code depot of Oracle tuning scripts.



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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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