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Oracle force index tips

Oracle Tips by Burleson Consulting
October 31,  2008

Question:  I want to use Oracle to force index usage for a SQL.  I've tried an index hint, but I still need to force index as the hint is ignored.  What is the trick in Oracle force index?

Answer: Given accurate CBO statistics, the Oracle optimizer will also choose the "best" index, and it's not always a good idea to force a specific index unless you are sure that the index will always be the fastest path to your rows.

The easiest way to force index usage is with the index hint.  When forcing an index, always use the table alias whenever you have a query that specifies an alias. For example, the following query will force the use of the dept_idx index because the emp table is aliased with ?e?:

select /*+ index(e,dept_idx) */ * from emp e;

Also, never reference the schema name in an index hint   Index hints will be ignored when the schema owner is specified in the hint. For example, the following hint will be ignored and it will not force the dept_idx index:

select /*+ index(scott.emp,dept_idx) */ * from emp;
 

 


 

 

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