Optimizer Hints

 A hint is a comment in a SQL statement that acts as an instruction to the optimizer.

Sometimes the application designer, who has more information about a particular application's data than is available to the optimizer, can choose a more effective way to run a SQL statement. The application designer can use hints in SQL statements to specify how the statement should be run. The following examples illustrate the use of hints.

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Example 7-2 Execution Plan for SELECT with FIRST_ROWS Hint

Suppose that your interactive application runs a query that returns 50 rows. This application initially fetches only the first 25 rows of the query to present to the end user. You want the optimizer to generate a plan that gets the first 25 records as quickly as possible so that the user is not forced to wait. You can use a hint to pass this instruction to the optimizer as shown in the SELECT statement and AUTOTRACE output in the following example:

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SELECT /*+ FIRST_ROWS(25) */ employee_id, department_id
FROM   hr.employees
WHERE  department_id > 50;

------------------------------------------------------------------------
| Id | Operation                    | Name              | Rows | Bytes
------------------------------------------------------------------------
|  0 | SELECT STATEMENT             |                   | 26   | 182
|  1 |  TABLE ACCESS BY INDEX ROWID | EMPLOYEES         | 26   | 182
|* 2 |   INDEX RANGE SCAN           | EMP_DEPARTMENT_IX |      |
------------------------------------------------------------------------

In this example, the execution plan shows that the optimizer chooses an index on the employees.department_id column to find the first 25 rows of employees whose department ID is over 50. The optimizer uses the rowid retrieved from the index to retrieve the record from the employees table and return it to the client. Retrieval of the first record is typically almost instantaneous.

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Example 7-3 Execution Plan for SELECT with No Hint

Assume that you execute the same statement, but without the optimizer hint:

SELECT employee_id, department_id
FROM   hr.employees
WHERE  department_id > 50;
 
------------------------------------------------------------------------
| Id | Operation              | Name              | Rows | Bytes | Cos
------------------------------------------------------------------------
|  0 | SELECT STATEMENT       |                   | 50   | 350   |
|* 1 |  VIEW                  | index$_join$_001  | 50   | 350   |
|* 2 |   HASH JOIN            |                   |      |       |
|* 3 |    INDEX RANGE SCAN    | EMP_DEPARTMENT_IX | 50   | 350   |
|  4 |    INDEX FAST FULL SCAN| EMP_EMP_ID_PK     | 50   | 350   |

In this case, the execution plan joins two indexes to return the requested records as fast as possible. Rather than repeatedly going from index to table as in Example 7-2, the optimizer chooses a range scan of EMP_DEPARTMENT_IX to find all rows where the department ID is over 50 and place these rows in a hash table. The optimizer then chooses to read the EMP_EMP_ID_PK index. For each row in this index, it probes the hash table to find the department ID.

In this case, the database cannot return the first row to the client until the index range scan of EMP_DEPARTMENT_IX completes. Thus, this generated plan would take longer to return the first record. Unlike the plan in Example 7-2, which accesses the table by index rowid, the plan uses multiblock I/O, resulting in large reads. The reads enable the last row of the entire result set to be returned more rapidly.

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