/*+ hint */All hints except /*+ rule */ cause the CBO to be used. Therefore, it is good practise to analyze the underlying tables if hints are used (or the query is fully hinted.
/*+ hint(argument) */
/*+ hint(argument-1 argument-2) */
There should be no schema names in hints. Hints must use aliases if alias names are used for table names. So the following is wrong:
select /*+ index(scott.emp ix_emp) */ from scott.emp emp_aliasbetter
select /*+ index(emp_alias ix_emp) */ ... from scott.emp emp_aliasWhy using hints
It is a perfect valid question to ask why hints should be used. Oracle comes with an optimizer that promises to optimize a query's execution plan. When this optimizer is really doing a good job, no hints should be required at all.
Sometimes, however, the characteristics of the data in the database are changing rapidly, so that the optimizer (or more accuratly, its statistics) are out of date. In this case, a hint could help.
It must also be noted, that Oracle allows to lock the statistics when they look ideal which should make the hints meaningless again.
Hints can be categorized as follows:
- Hints for Optimization Approaches and Goals,
- Hints for Access Paths, Hints for Query Transformations,
- Hints for Join Orders,
- Hints for Join Operations,
- Hints for Parallel Execution,
- Additional Hints
