How to enforce index

Question

Hi Guys,
How to enforce index ?
Say for example,
select max(dt) from emp
emp table has an index called 'id1'.
I want to enforce this index for the above query. How ?
Inputs are welcome !

Answer 1

Your index on ID is worthless for this query.
You would have to add an index on your DT column. Even then, the index will only be used if you have a NOT NULL constraint on the column (or add "and dt is not null" to your query):
SQL> explain plan for
  2  select min(dt), max(dt) from emp;
Explained.
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |             |     1 |     8 |   194 |
|   1 |  SORT AGGREGATE      |             |     1 |     8 |       |
|   2 |   TABLE ACCESS FULL  | EMP         | 91000 |   710K|   194 |
SQL> alter table emp modify (dt not null);
Table altered.
SQL> explain plan for
  2  select min(dt), max(dt) from emp;
Explained.
| Id  | Operation             |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT      |             |     1 |     8 |    39 |
|   1 |  SORT AGGREGATE       |             |     1 |     8 |       |
|   2 |   INDEX FAST FULL SCAN| EMP_DT      | 91000 |   710K|    39 |
---------------------------------------------------------------------Still, even without an index a full table scan of 91000 rows takes < 1 second even on my laptop. How are you getting 21 seconds?