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Improving SQL execution plans

Intelligent buffer segregation improves overall execution speed by reducing buffer gets, but there are also some other important reasons to use multiple blocksizes. 

 

In general, the Oracle CBO is unaware of buffer details, except when the optimizer_index_caching parameter is set where using multiple data buffers will not impact SQL execution plans.  When data using the new cpu_cost parameter in Oracle10g, the Oracle SQL optimizer builds the SQL plan decision tree based on the execution plan that will have the lowest estimated CPU cost. For example, if a 32k data buffer is implemented for the index tablespaces, the DBA can ensure that the indexes are cached for optimal performance and minimal logical I/O in range scans.

 

For example, if a database has 50 gigabytes of index space, a 60 gigabyte db_32k_cache_size can be defined and then the optimizer_index_caching parameter can be set to 100, telling the SQL optimizer that all of the Oracle indexes reside in RAM.  When Oracle makes the index versus table scan decision, knowing that the index nodes are in RAM will greatly influence the optimizer because the CBO knows that a logical I/O is often 100 times faster than a physical disk read.

 

In sum, moving Oracle indexes into a fully cached 32k buffer will ensure that Oracle favors index access, reducing unnecessary full table scans and greatly reducing logical I/O because adjacent index nodes will reside within the larger, 32k block.

 

SEE CODE DEPOT FOR FULL SCRIPTS


This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

 

 

   

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