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no_cpu_costing tips

Oracle Database Tips by Donald BurlesonMay 11, 2016

Question: When do I use the no_cpu_costing hint on a SQL statement.  I am on 12c and I understand that the no_cpu_costing hint can improve SQL performance.  Can you explain how the no_cpu_costing hint works?

Answer:  The no_cpu_costing hint was introduced to turn off the including of CPU costs in the decision tree for choosing the "best" execution plan.  This hint turns off CPU costing for the SQL statement. CBO then uses the IO cost model, which measures everything in single-block reads and ignores CPU cost.

Make sure to read these important notes on the optimizer cost model.

Oracle introduced CPU costing as the default _optimizer_cost_model=cpu because most Oracle database are CPU-bound and you want to optimizer SQL to minimize CPU costs.  However, there are cases where you want to use I/O costs when optimizing a SQL statement (this can be done system-wide by setting _optimizer_cost_model=io). 

I/O-bound databases, (especially older 32-bit databases), may want to utilize I/O-based SQL costing.  The default optimizer costing in Oracle10g and beyond is CPU, and it can be changed to IO costing by using these techniques:

 SQL> alter system set optimizer_index_cost_adj=100 scope=both;

SQL> alter session set “_optimizer_cost_model=io; 

Set init.ora hidden parameter for a permanent change:

SQL> alter system set "_optimizer_cost_model"=io scope=both;

There are many documented cases in Oracle 12c and beyond where you want to improve the speed of SQL statements by turning-off CPU costing at the statement level:

select /*+ no_cpu_costing */ col1 from mytab;


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