How to make an index use in a query execution

Question

Hi,
I have the below query for which ename column has an index. As of my knowledge below queries 1st and 2st will not use index. Hence i used the 3rd statement and that too its not using the index. Finally i used the 4th query, but even the 4th query is not using the index. Then how do i make this query to use my index??? Do i need to create a function based index for this??? Is that the final option????
1. select * from emp where ename !='BH' ;
2. select * from emp where ename <> 'BH';
3. select * from emp where ename not in ('BH');
4. select * from emp where ename < 'BH' or ename > 'BH';
Regards,
007
Edited by: 007 on Jun 6, 2013 7:56 AM
Edited by: 007 on Jun 6, 2013 8:06 AM
Edited by: 007 on Jun 6, 2013 8:06 AM
Edited by: 007 on Jun 6, 2013 8:06 AM
Edited by: 007 on Jun 6, 2013 8:12 AM

Answer 1

Sorry 007, I really thought you were posting a trick question as on the OCP tests.
Anyway, as Justin mentioned, if you have an index on ename, it may be used when doing a comparison predicate statement with the ename value.
What it depends on are several other things: stats, how many rows in the table, use of an index hint, etc.
Rather than questioning the group on this, why not just turn on autotrace and run the query for the different scenarios.
The output will show you if it used the index, number of rows returned, blocks read, etc.
SQL> create table emp (ename  varchar2(40));
Table created.
SQL> insert into emp select username from sys.dba_users;
25 rows created.
SQL> commit;
Commit complete.
SQL> set autotrace on
SQL> select * from emp where ename != 'SYSTEM';
Execution Plan
Plan hash value: 2951343571
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |           |    24 |   528 |     1   (0)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | ENAME_IDX |    24 |   528 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("ENAME"<>'SYSTEM')As you can see, the above used an index, even though there were only 25 rows in the table.
You can test each of your scenarios, one by one, including use of a hint.