Why it is taking full table scan when index is available

Question

Hi all,
I am facing a strange problem with index.
I created index on a column like
1. CREATE INDEX ASSETS_ARTIST_IDX2 ON ASSETS(artist).
SELECT asset.artist AS NAME FROM ASSETS asset WHERE asset.artist LIKE 'J%'
Explain Plan : INDEX RANGE SCAN
2. CREATE INDEX ASSETS_ARTIST_IDX2 ON ASSETS(UPPER (TRANSLATE (artist, ',;:"', ' ')));
SELECT asset.artist AS NAME FROM ASSETS asset WHERE UPPER (TRANSLATE (artist, ',;:"', ' ')) LIKE 'J%'
          Explain Plan : TABLE ACCESS FULL
first time it is taking index range scan,but in second situation scaning full table.Please let me know how to avoid the full table scan.
Regards,
Rajasekhar

Answer 1

Actually, I misseunderstood damorgan' s statement about NULL and OP did not provide table definition. So based on FTS I would say it is more likely column is NULLable, otherwise I would expect INDEX FAST SCAN:
SQL> create table emp1 as select * from emp
  2  /
Table created.
SQL> create index emp1_idx1 on emp1(empno)
  2  /
Index created.
SQL> exec dbms_stats.gather_table_stats('SCOTT','EMP1');
PL/SQL procedure successfully completed.
SQL> desc emp1
Name                                                                     Null?    Type
EMPNO                                                                             NUMBER(4)
ENAME                                                                             VARCHAR2(10)
JOB                                                                               VARCHAR2(9)
MGR                                                                               NUMBER(4)
HIREDATE                                                                          DATE
SAL                                                                               NUMBER(7,2)
COMM                                                                              NUMBER(7,2)
DEPTNO                                                                            NUMBER(2)
SQL> explain plan for
  2  select empno
  3  from emp1 where UPPER(TRANSLATE(empno, ',;:"', ' ')) LIKE '7%'
  4  /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 2226897347
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |     1 |     4 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP1 |     1 |     4 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter(UPPER(TRANSLATE(TO_CHAR("EMPNO"),',;:"',' ')) LIKE '7%')
13 rows selected.
SQL> alter table emp1 modify empno not null
  2  /
Table altered.
SQL> explain plan for
  2  select empno
  3  from emp1 where UPPER(TRANSLATE(empno, ',;:"', ' ')) LIKE '7%'
  4  /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 2850860361
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |           |     1 |     4 |     1   (0)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | EMP1_IDX1 |     1 |     4 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter(UPPER(TRANSLATE(TO_CHAR("EMPNO"),',;:"',' ')) LIKE '7%')
13 rows selected.
SQL> SY.