Does GROUP BY force oracle go for sorting?

Question

Version : 10.2
I usually rewrite all queries with DISTINCT clause to GROUP BY to avoid sorting in the temporary tablespace. But, doesn't GROUP BY queries go for sorting?

Answer 1

Hi,
I am on 10.2.0.1
SQL> alter system flush shared_pool;
System altered.
SQL> alter system flush buffer_cache;
System altered.
SQL> select deptno from emp group by deptno;
    DEPTNO
        30
        20
        10
Execution Plan
Plan hash value: 4067220884
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     3 |     9 |     4  (25)| 00:00:01 |
|   1 |  HASH GROUP BY     |      |     3 |     9 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP  |    14 |    42 |     3   (0)| 00:00:01 |
Statistics
        335  recursive calls
          0  db block gets
         64  consistent gets
         21  physical reads
          0  redo size
        456  bytes sent via SQL*Net to client
        381  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          5  sorts (memory)
          0  sorts (disk)
          3  rows processed
SQL> alter system flush shared_pool;
System altered.
SQL>  alter system flush buffer_cache;
System altered.
SQL> select distinct deptno from emp;
    DEPTNO
        30
        20
        10
Execution Plan
Plan hash value: 3709190377
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     3 |     9 |     4  (25)| 00:00:01 |
|   1 |  HASH UNIQUE       |      |     3 |     9 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP  |    14 |    42 |     3   (0)| 00:00:01 |
Statistics
        754  recursive calls
          0  db block gets
        145  consistent gets
         31  physical reads
          0  redo size
        456  bytes sent via SQL*Net to client
        381  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         17  sorts (memory)
          0  sorts (disk)
          3  rows processedhmm,group by does sort but distinct seems to be more expensive w.r.t sorting.