Basic Question - Update - Usage of index

Question

Gurus,
I have a basic question. As per my knowledge, an index will speed up the process while we are selecting the data. If we are doing some DML operations (especially Update), do we need the index to speed up the process eventhough the indexed column is in the where condition?
Regards
Edited by: Sarma12 on Apr 17, 2012 5:59 AM

Answer 1

Have you tried setting up a test scenario? For example:
SQL> CREATE TABLE test AS SELECT 1 num FROM DUAL CONNECT BY LEVEL <= 1000;
Table created.
SQL> UPDATE test SET NUM = 99 WHERE ROWNUM = 1;
1 row updated.
SQL> COMMIT;
Commit complete.
SQL> UPDATE /*+gather_plan_statistics*/
  2         test
  3  SET    num = 2
  4  WHERE  num = 99
  5  ;
1 row updated.
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
SQL_ID  96wt4ddwy0sa5, child number 0
UPDATE /*+gather_plan_statistics*/        test SET    num = 2 WHERE
num = 99
Plan hash value: 3859524075
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   0 | UPDATE STATEMENT   |      |      1 |        |      0 |00:00:00.01 |       7 |
|   1 |  UPDATE            | TEST |      1 |        |      0 |00:00:00.01 |       7 |
|*  2 |   TABLE ACCESS FULL| TEST |      1 |      1 |      1 |00:00:00.01 |       4 |
Predicate Information (identified by operation id):
   2 - filter("NUM"=99)
Note
   - dynamic sampling used for this statement (level=2)
24 rows selected.
SQL> rollback;
Rollback complete.
SQL> CREATE INDEX test_x1 ON test(num);
Index created.
SQL> UPDATE /*+gather_plan_statistics*/
  2         test
  3  SET    num = 2
  4  WHERE  num = 99
  5  ;
1 row updated.
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
SQL_ID  96wt4ddwy0sa5, child number 0
UPDATE /*+gather_plan_statistics*/        test SET    num = 2 WHERE
num = 99
Plan hash value: 734435536
| Id  | Operation         | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   0 | UPDATE STATEMENT  |         |      1 |        |      0 |00:00:00.01 |       9 |
|   1 |  UPDATE           | TEST    |      1 |        |      0 |00:00:00.01 |       9 |
|*  2 |   INDEX RANGE SCAN| TEST_X1 |      1 |      1 |      1 |00:00:00.01 |       2 |
Predicate Information (identified by operation id):
   2 - access("NUM"=99)
Note
   - dynamic sampling used for this statement (level=2)
24 rows selected.So yes, an index may be used in DML.