Query Tuning! - Need your help

Question

Hi Guys,
I have got a problem in Query Tuning.
I have a query, in which I am accesing tables from different users. Example:
2 tables are from my own schema, 3 tables from other users and 3 tables from yet another user.
I am using DISTINCT on this select statement. The 1st two columns in teh select list are concatenated with each other.
There is yet another select in the from clause to get few other records from other tables.
Thee query returns 163 records and takes around 14 seconds.
I referred PLAN_TABLE, its showing NESTED LOOPS and CARTESIAN JOIN in the SORT OPERATION.
Can anyone of you please suggest areas where I can concentrate?
Thanks in advance
Himanshu Dabir

Answer 1

DISTINCT always do an inherent sort.
SQL> EXPLAIN PLAN FOR
  2   SELECT DISTINCT principal_due,outstand_balance
  3     FROM outstanding_ledger
  4    WHERE dnum=1030003931
  5  /
Explained.
SQL> SET LINE 200
SQL> SELECT * FROM TABLE(dbms_xplan.display)
  2  /
PLAN_TABLE_OUTPUT
Plan hash value: 1765098853
| Id  | Operation                    | Name                    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |                         |    17 |   272 |     1 (100)| 00:00:01 |
| 1 | SORT UNIQUE | | 17 | 272 | 1 (100)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| OUTSTANDING_LEDGER      |    17 |   272 |            |          |
|*  3 |    INDEX SKIP SCAN           | DLINE_COM_DDATE_DNUM_PK |    17 |       |            |          |
PLAN_TABLE_OUTPUT
Predicate Information (identified by operation id):
   3 - access("DNUM"=1030003931)
       filter("DNUM"=1030003931)
16 rows selected.
SQL> DELETE plan_table
  2  /
4 rows deleted.
SQL> commit
  2  /
Commit complete.
SQL> EXPLAIN PLAN FOR
  2   SELECT  principal_due,outstand_balance
  3     FROM outstanding_ledger
  4    WHERE dnum=1030003931
  5  /
Explained.
SQL> SELECT * FROM TABLE(dbms_xplan.display)
  2  /
PLAN_TABLE_OUTPUT
Plan hash value: 3157896501
| Id  | Operation                   | Name                    | Rows  | Bytes | Cost (%CPU)|
|   0 | SELECT STATEMENT            |                         |    17 |   272 |     0   (0)|
|   1 |  TABLE ACCESS BY INDEX ROWID| OUTSTANDING_LEDGER      |    17 |   272 |            |
|*  2 |   INDEX SKIP SCAN           | DLINE_COM_DDATE_DNUM_PK |    17 |       |            |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - access("DNUM"=1030003931)
       filter("DNUM"=1030003931)
15 rows selected.
SQL>
Can anyone of you please suggest areas where I can concentrate?Paste yours plan with updated stats.
Khurram