How Can I increase performance of Query with Distinct Keyword

Question

Dear Experts,
In my Query when I execute this without Distinct it give result very soon,
But when I excute this with Distinct its performance is very slow is there any option for increase the performance with Distinct .
I have to use Distinct according to our rewuirement.

Answer 1

neither DISTINCT nor GROUP BY do sort. If you need to sort, use ORDER BY. Slight correction, they do a sort, but your results are not guarenteed to be sorted.
The overhead of a sort is incurred, if your results is large enough (my test below isn't) you will be sorting to disk. Which as we all know can be very slow.
9i:
SQL> SET TRIMSPOOL ON
SQL> CREATE TABLE D (A VARCHAR2(2));
Table created.
SQL> INSERT INTO D (A) SELECT DBMS_RANDOM.STRING('U',2) FROM DUAL CONNECT BY LEVEL <= 5000;
5000 rows created.
SQL> COMMIT;
Commit complete.
SQL> SET AUTOTRACE TRACEONLY EXPLAIN;
SQL> SELECT DISTINCT A FROM D;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (UNIQUE)
   2    1     TABLE ACCESS (FULL) OF 'D'
SQL> SELECT A FROM D GROUP BY A;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (GROUP BY)
   2    1     TABLE ACCESS (FULL) OF 'D'
SQL> SELECT DISTINCT A FROM D ORDER BY A;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (UNIQUE)
   2    1     TABLE ACCESS (FULL) OF 'D'
SQL> SPOOL OFF10g
SQL> SET TRIMSPOOL ON
SQL> CREATE TABLE D (A VARCHAR2(2));
Table created.
SQL> INSERT INTO D (A) SELECT DBMS_RANDOM.STRING('U',2) FROM DUAL CONNECT BY LEVEL <= 5000;
5000 rows created.
SQL> COMMIT;
Commit complete.
SQL> SET AUTOTRACE TRACEONLY EXPLAIN;
SQL> SELECT DISTINCT A FROM D;
Execution Plan
Plan hash value: 3079699766
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |  5000 | 15000 |     7  (29)| 00:00:01 |
|   1 |  HASH UNIQUE       |      |  5000 | 15000 |     7  (29)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| D    |  5000 | 15000 |     5   (0)| 00:00:01 |
Note
   - dynamic sampling used for this statement
SQL> SELECT A FROM D GROUP BY A;
Execution Plan
Plan hash value: 2712634873
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |  5000 | 15000 |     7  (29)| 00:00:01 |
|   1 |  HASH GROUP BY     |      |  5000 | 15000 |     7  (29)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| D    |  5000 | 15000 |     5   (0)| 00:00:01 |
Note
   - dynamic sampling used for this statement
SQL> SELECT DISTINCT A FROM D ORDER BY A;
Execution Plan
Plan hash value: 1773491675
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |  5000 | 15000 |     8  (38)| 00:00:01 |
|   1 |  SORT UNIQUE       |      |  5000 | 15000 |     7  (29)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| D    |  5000 | 15000 |     5   (0)| 00:00:01 |
Note
   - dynamic sampling used for this statement
SQL> SPOOL OFF