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Oracle issues bulletin on SQL predicate evaluation

May 31,2005

MOSC bulletin 276877.1 has some important information on how the Oracle SQL optimizer evaluates predicates in the WHERE clause:


Some of the things that influence the order of application of the predicates are:


- Their position in the WHERE clause.

- The kind of operators involved

- Automatic Datatype conversion.

- The presence of bind variables

- Joins

- The optimizer mode (Rule or Cost)

- The cost model (IO or CPU)

- The Query Rewrite feature

- Distributed operations

- View merging 

- Query Un-nesting

- Other Documented and Undocumented Parameters

- Hints


It is also important to note the bug in Oracle9i where adjusting optimizer_index_cost_adj will cause CPU costs to be considered in execution plans.  More important, setting the SQL optimizer cost model (CPU or IO) will also affect the way that the optimizer evaluates predicates:


Oracle 10g has recognized this trend toward CPU-based optimization by providing you with the ability to choose CPU-based or I/O-based costing during SQL optimization (the 10g default is CPU-costing).  In Oracle10g, system stats are gathered by default, and in Oracle9i the DBA must manually execute the dbms_stat.gather_system_stats package to get CBO statistics.

alter session set "_optimizer_cost_model"=io; 

alter session set "_optimizer_cost_model"=cpu; 


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