SQL Tuning - Not understanding the explain plan.

Question

Hi All,
I am using 11g R2 and I have 2 questions about tuning a query.
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for 64-bit Windows: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - ProductionMy parameters relevant to the optimizer are :
NAME                                 TYPE        VALUE
optimizer_capture_sql_plan_baselines boolean     FALSE
optimizer_dynamic_sampling           integer     2
optimizer_features_enable            string      11.2.0.3
optimizer_index_caching              integer     0
optimizer_index_cost_adj             integer     100
optimizer_mode                       string      ALL_ROWS
optimizer_secure_view_merging        boolean     TRUE
optimizer_use_invisible_indexes      boolean     FALSE
optimizer_use_pending_statistics     boolean     FALSE
optimizer_use_sql_plan_baselines     boolean     TRUEThe query I would like to execute is pretty simple. It just returns some exceptions with a filter.
SELECT ERO.DVC_EVT_ID,  E.DVC_EVT_DTTM
FROM D1_DVC_EVT E,  D1_DVC_EVT_REL_OBJ ERO
WHERE ERO.MAINT_OBJ_CD = 'D1-DEVICE'
AND ERO.PK_VALUE1 = :H1
AND ERO.DVC_EVT_ID = E.DVC_EVT_ID
AND E.DVC_EVT_TYPE_CD IN ('END-GSMLOWLEVEL-EXCP-SEV-1', 'STR-GSMLOWLEVEL-EXCP-SEV-1')
ORDER BY E.DVC_EVT_DTTM DESC;The execution plan is the following :
Plan hash value: 3627978539
| Id  | Operation                            | Name               | Starts | E-Rows |E-Bytes| Cost (%CPU)| Pstart| Pstop | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |
|   0 | SELECT STATEMENT                     |                    |      1 |        |       |  7131 (100)|       |       |   1181 |00:00:17.17 | 8627 |   2978 |       |       |          |
|   1 |  SORT ORDER BY                       |                    |      1 |   3137 |   275K|  7131   (1)|       |       |   1181 |00:00:17.17 | 8627 |   2978 | 80896 | 80896 |71680  (0)|
|   2 |   NESTED LOOPS                       |                    |      1 |        |       |            |       |       |   1181 |00:00:17.16 | 8627 |   2978 |       |       |          |
|   3 |    NESTED LOOPS                      |                    |      1 |   3137 |   275K|  7130   (1)|       |       |   2058 |00:00:08.09 | 6709 |   1376 |       |       |          |
|   4 |     TABLE ACCESS BY INDEX ROWID      | D1_DVC_EVT_REL_OBJ |      1 |   3137 |   125K|   845   (1)|       |       |   2058 |00:00:04.37 |  820 |    799 |       |       |          |
|*  5 |      INDEX RANGE SCAN                | D1T404S0           |      1 |   3137 |       |    42   (0)|       |       |   2058 |00:00:00.08 |   27 |     23 |       |       |          |
|   6 |     PARTITION RANGE ITERATOR         |                    |   2058 |      1 |       |     1   (0)|   KEY |   KEY |   2058 |00:00:03.69 | 5889 |    577 |       |       |          |
|*  7 |      INDEX UNIQUE SCAN               | D1T400P0           |   2058 |      1 |       |     1   (0)|   KEY |   KEY |   2058 |00:00:03.66 | 5889 |    577 |       |       |          |
|*  8 |    TABLE ACCESS BY GLOBAL INDEX ROWID| D1_DVC_EVT         |   2058 |      1 |    49 |     2   (0)| ROWID | ROWID |   1181 |00:00:09.05 | 1918 |   1602 |       |       |          |
Peeked Binds (identified by position):
   1 - (VARCHAR2(30), CSID=178): '271792300706'
Predicate Information (identified by operation id):
   5 - access("ERO"."PK_VALUE1"=:H1 AND "ERO"."MAINT_OBJ_CD"='D1-DEVICE')
       filter("ERO"."MAINT_OBJ_CD"='D1-DEVICE')
   7 - access("ERO"."DVC_EVT_ID"="E"."DVC_EVT_ID")
   8 - filter(("E"."DVC_EVT_TYPE_CD"='END-GSMLOWLEVEL-EXCP-SEV-1' OR "E"."DVC_EVT_TYPE_CD"='STR-GSMLOWLEVEL-EXCP-SEV-1'))So as you can see, row 8, I have a TABLE ACCESS BY GLOBAL INDEX ROWID. But what I am failling to see is how Oracle display a TABLE ACCESS BY GLOBAL INDEX ROWID without using any index. As thought that Oracle always got a ROWID because of an index.
I also have an index on the column DVC_EVT_TYPE_CD in my table (row 8 of the predicate information section)
And finally what would be your suggestion to improve the runtime of this query...
Many thanks.

Answer 1

if I read the rowsource statistics information and the plan correctly then the table access in step 4 on D1_DVC_EVT_REL_OBJ needs 4.37 sec and accesses 820 buffers. If you had an index D1_DVC_EVT_REL_OBJ(MAINT_OBJ_CD, PK_VALUE1, DVC_EVT_ID) I assume that this table access could be avoided. To avoid the table access on D1_DVC_EVT an index on D1_DVC_EVT(DVC_EVT_ID, DVC_EVT_TYPE_CD, DVC_EVT_DTTM) should be sufficient.
I think these indexes would improve the performance of the given query but of course they would have a negative impact on DML performance on the table and they could influence other queries on the table.