Inappropriate index used

Question

Hi All,
I have a table with following structure initially without any data.
Name                                                  Null?    Type
DTL_MSG_ID                                            NOT NULL NUMBER(9)
MODULE_TYPE                                           NOT NULL VARCHAR2(2 CHAR)
MSG_ID                                                NOT NULL NUMBER(9)
WHSE                                                  NOT NULL VARCHAR2(3 CHAR)
MSG_TYPE                                                       VARCHAR2(10 CHAR)
MSG                                                            VARCHAR2(1000 CHAR)
MSG_OBJ                                                        BLOB
CREATE_DATE_TIME                                               DATE
MOD_DATE_TIME                                                  DATE
MISC1                                                          VARCHAR2(20 CHAR)
MISC2                                                          VARCHAR2(20 CHAR)
VERSION_ID                                            NOT NULL NUMBER(6)
Below is the list of index created on table columns.
SQL> select index_name,table_name,column_name,column_position from user_ind_columns where table_name='E_WMS_EVNT_MSG_DTL' order by 1,4;
INDEX_NAME                     TABLE_NAME                     COLUMN_NAME                              COLUMN_POSITION
E_WMS_EVNT_MSG_DTL_IND_1       E_WMS_EVNT_MSG_DTL             MSG_ID                                                 1
FK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             MSG_ID                                                 1
FK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             MODULE_TYPE                                            2
FK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             WHSE                                                   3
FK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             DTL_MSG_ID                                             4
PK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             DTL_MSG_ID                                             1
PK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             MODULE_TYPE                                            2
PK_E_WMS_EVNT_MSG_DTL          E_WMS_EVNT_MSG_DTL             WHSE                                                   3
8 rows selected.
Generate Explain plan using order by cluase in query
SQL> explain plan for SELECT wmsevntmsg0_.module_type AS module2_107_, wmsevntmsg0_.dtl_msg_id AS dtl1_107_, wmsevntmsg0_.msg_type AS msg5_107_, wmsevntmsg0_.msg AS msg107_, wmsevntmsg0_.msg_id AS msg4_107_, wmsevntmsg0_.msg_obj AS msg7_107_, wmsevntmsg0_.create_date_time AS create8_107_, wmsevntmsg0_.mod_date_time AS mod9_107_, wmsevntmsg0_.whse AS whse107_, wmsevntmsg0_.misc1 AS misc10_107_, wmsevntmsg0_.misc2 AS misc11_107_
  2  FROM e_wms_evnt_msg_dtl wmsevntmsg0_ WHERE wmsevntmsg0_.msg_id = :1 AND wmsevntmsg0_.module_type = :2 AND wmsevntmsg0_.whse = :3 ORDER BY wmsevntmsg0_.msg_id,wmsevntmsg0_.dtl_msg_id ASC
  3  ;
Plan is using the PK index to fetch data.
Plan hash value: 2070795294
| Id  | Operation                   | Name                  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |                       |   514 |   306K|  6425   (1)| 00:01:18 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| E_WMS_EVNT_MSG_DTL    |   514 |   306K|  6425   (1)| 00:01:18 |
|*  2 |   INDEX FULL SCAN           | PK_E_WMS_EVNT_MSG_DTL |   253K|       |   177   (2)| 00:00:03 |
Explain plan without using Order by in query.
SQL> explain plan for SELECT wmsevntmsg0_.module_type AS module2_107_, wmsevntmsg0_.dtl_msg_id AS dtl1_107_, wmsevntmsg0_.msg_type AS msg5_107_, wmsevntmsg0_.msg AS msg107_, wmsevntmsg0_.msg_id AS msg4_107_, wmsevntmsg0_.msg_obj AS msg7_107_, wmsevntmsg0_.create_date_time AS create8_107_, wmsevntmsg0_.mod_date_time AS mod9_107_, wmsevntmsg0_.whse AS whse107_, wmsevntmsg0_.misc1 AS misc10_107_, wmsevntmsg0_.misc2 AS misc11_107_
FROM e_wms_evnt_msg_dtl wmsevntmsg0_ WHERE wmsevntmsg0_.msg_id = 115952 AND wmsevntmsg0_.module_type = '10' AND wmsevntmsg0_.whse = '61'  2  ;
I believe its now using the correct index to fetch data.
Plan hash value: 461238093
| Id  | Operation                   | Name                  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |                       |     1 |   611 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| E_WMS_EVNT_MSG_DTL    |     1 |   611 |     1   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | FK_E_WMS_EVNT_MSG_DTL |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------Please suggest, I am wondering why oracle using PK index in first scenario. Table has the updated stats.
Thanks in Advanced.
Bhupinder

Answer 1

Hi Oscar,
Thanks a lot for your reply.
But the problem here is why Oracle is not using the index I created for all the columns i WHERE condition, instead it is looking for data from Primary Key index.
Regards,
Bhupinder