Disabling Indexes

Question

Hi,
in 10g R2,
it is said that the following WHERE clause will disable index :
WHERE ''|| column >=valueBut when I use it the explain plan does not change :
SQL> EXPLAIN PLAN FOR
  2  select EMPLID, NAME FROM sysadm.PS_NAMES WHERE EMPLID >= 'PA001';
Explained.
SQL> SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
PLAN_TABLE_OUTPUT
Plan hash value: 170795870
| Id  | Operation            | Name     | Rows  | Bytes | Cost (%CPU)| Time
|
PLAN_TABLE_OUTPUT
|   0 | SELECT STATEMENT     |          |   164 |  5576 |    71   (2)| 00:00:01
|
|*  1 |  INDEX FAST FULL SCAN| PS0NAMES |   164 |  5576 |    71   (2)| 00:00:01
|
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter("EMPLID">='PA001')
13 rows selected.And with that trick :
SQL>  EXPLAIN PLAN FOR
  2   select EMPLID, NAME FROM sysadm.PS_NAMES WHERE ''||EMPLID >= 'PA001';
Explained.
SQL> SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
PLAN_TABLE_OUTPUT
Plan hash value: 170795870
| Id  | Operation            | Name     | Rows  | Bytes | Cost (%CPU)| Time
|
PLAN_TABLE_OUTPUT
|   0 | SELECT STATEMENT     |          |   164 |  5576 |    71   (2)| 00:00:01
|
|*  1 |  INDEX FAST FULL SCAN| PS0NAMES |   164 |  5576 |    71   (2)| 00:00:01
|
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter(''||"EMPLID">='PA001')
13 rows selected.Where am I wrong ?
Thank you.

Answer 1

The reason why it does not work for you is, I suspect, because you have an index on all the columns you use in your query. In that one instance, there is absolutely no need to look in the table at all as all the information is available in the index.
Judging from your explain plans, that is exactly what is happening in your case (there is no Table lookup once the index has been accessed, you see!).
Eg:
SQL> create table test1 (col1 number,
  2                      col2 varchar2(20),
  3                      col3 number,
  4                      col4 number,
  5                      col5 number,
  6                      constraint test1_pk primary key (col1));
Table created.
SQL>              
SQL> insert into test1
  2  select level, 'name_'||level, level, level + 1, level + 2
  3  from dual
  4  connect by level <= 10000;
10000 rows created.
SQL>
SQL> commit;
Commit complete.
SQL>
SQL> create index test1_idx on test1 (col1, col2);
Index created.
SQL>
SQL> explain plan for
  2  select col1, col2
  3  from   test1
  4  where  col2 >= ''||'name_9990';
Explained.
SQL>
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 1755968961
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |           |    10 |   250 |   100  (53)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | TEST1_IDX |    10 |   250 |   100  (53)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - access("COL2">='name_9990')
       filter("COL2">='name_9990')
Note
   - dynamic sampling used for this statement
18 rows selected.
SQL>
SQL> explain plan for
  2  select col1, col2
  3  from   test1
  4  where  col2 >= ''||'name_9990';
Explained.
SQL>
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 1755968961
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |           |    10 |   250 |   100  (53)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | TEST1_IDX |    10 |   250 |   100  (53)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - access("COL2">='name_9990')
       filter("COL2">='name_9990')
Note
   - dynamic sampling used for this statement
18 rows selected.
SQL>
SQL> explain plan for
  2  select col1, col2
  3  from   test1
  4  where  col3 >= 9990;
Explained.
SQL>
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT                                                              
Plan hash value: 3852271815                
| Id  | Operation         | Name  | Rows  | Bytes | Cost (%CPU)| Time     |    
|   0 | SELECT STATEMENT  |       |    11 |   418 |   152  (35)| 00:00:01 |    
|*  1 |  TABLE ACCESS FULL| TEST1 |    11 |   418 |   152  (35)| 00:00:01 |    
Predicate Information (identified by operation id):                            
   1 - filter("COL3">=9990)                                                    
Note                                                                           
   - dynamic sampling used for this statement                                  
17 rows selected.
SQL>
SQL> drop table test1;
Table dropped.