How to compile the hint to force selection statement to use index

Question

Hello expert,
will you please tell me how to compile the hint to force selection statement to use index?
Many Thanks,

Answer 1

Not sure what you mean by compile, but hint is enclosed in /*+ hint */. Index hint is INDEX(table_name,index_name). For example:
SQL> explain plan for
  2  select * from emp
  3  /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 3956160932
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |    14 |   546 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   546 |     3   (0)| 00:00:01 |
8 rows selected.
SQL> explain plan for
  2  select /*+ index(emp,pk_emp) */ *
  3  from emp
  4  /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 4170700152
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |        |    14 |   546 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |    14 |   546 |     2   (0)| 00:00:01 |
|   2 |   INDEX FULL SCAN           | PK_EMP |    14 |       |     1   (0)| 00:00:01 |
9 rows selected.
SQL> Hint in the above example is forcing optimizer to use index which resul;ts in a bad execution plan. Most of the time optimizer does not need hints and chooses an optimal plan. In most cases sub-optimal plan is result of stale or incomplete statistics.
SY.