Wednesday, June 8, 2016

Using Index Hints in oracle

Hints :

Hints are used to give specific information that we know about our data and application, to Oracle. This further improves the performance of our system. There can be instances where the default optimizer may not be efficient for certain SQL statements. We can specify HINTS with the SQL statements, to improve the efficiency of those SQL statements. Hints should only be used as a last-resort if statistics were gathered and the query is still following a sub-optimal execution plan.

Example of the correct syntax for an index hint:

select /*+ index(TEST_IDX IDX_OS_USR) */ * from TEST_IDX;

If we alias the table (A in below case), you must use the alias in the index hint:

select /*+ index(A IDX_OS_USR) */ * from TEST_IDX A;

Note :

Oracle decides to use weather to use this hint or not, of oracle finds that it has faster execution plan without using hint it ignores it. You might think that an index may be helpfull and provide it as hint but oracle may still ignore it. In below case you can see hint being ignored.

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