OPT_PARAM Hint

Applies to:
Oracle Server – Enterprise Edition – Version: 10.2.0.1 to 10.2.0.3 – Release: 10.2 to 10.2
Information in this document applies to any platform.
Goal
This article is explains the new optimizer hint “OPT_PARAM” introduced in 10g R2.
Solution
“OPT_PARAM” is a new optimizer hint introduced in 10g Release 2. This hint behaves the same way as
setting a parameter (e.g, using alter session) except that the effect is for the statement only. The hint only works for optimizer parameters. Global parameters such as optimizer_features_enable are not covered but optimizer_features_enable specifically has its own hint:

/*+ optimizer_features_enable(‘9.2.0’) */

@For an INTERNAL list of usable parameters see: Note:986618.1 Parameters useable by OPT_PARAM hint
Hint Syntax
The syntax is:

/*+ opt_param( [,] ) */

parameter_name is the name of a parameter
parameter_value is its value.

If the parameter contains a numeric value, the parameter value has to be specified without quotes.
The hint can be used to set multiple parameters by repeating the hint, i.e.

/*+ opt_param( [,] )
opt_param( [,] )
*/

Basic Usage Example
For example, the following hint sets <> to ‘false’ when added to a statement:

/*+ opt_param(‘hash_join_enabled’,’false’) */

e.g.:
Without the hint:


SQL> select empno from emp e, dept d where e.ename=d.dname

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     8 |   160 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN         |      |     8 |   160 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    40 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |    28 |   280 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

With the hint the hash join is disabled choosing a different plan:


SQL> select /*+ opt_param('hash_join_enabled','false') */ empno 
from emp e, dept d where e.ename=d.dname;


----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |     8 |   160 |     8  (25)| 00:00:01 |
|   1 |  MERGE JOIN         |      |     8 |   160 |     8  (25)| 00:00:01 |
|   2 |   SORT JOIN         |      |     4 |    40 |     4  (25)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| DEPT |     4 |    40 |     3   (0)| 00:00:01 |
|*  4 |   SORT JOIN         |      |    28 |   280 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL| EMP  |    28 |   280 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------



Multiple Parameter Settings example
The OPT_PARAM hint can be specified more than once Time to adjust more than one parameter at once as follows:


/*+ OPT_PARAM('_always_semi_join' 'off')
      OPT_PARAM('_b_tree_bitmap_plans' 'false')
      OPT_PARAM('query_rewrite_enabled' 'false')
      OPT_PARAM('_new_initial_join_orders' 'false')
      OPT_PARAM('optimizer_dynamic_sampling' 1)
      OPT_PARAM('optimizer_index_cost_adj' 1) */

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