group by 替代distinct

SQL> select distinct employee_id,first_name from test1;

107 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 255531131

------------------------------------------------------------------------------------
| Id  | Operation	   | Name  | Rows  | Bytes |TempSpc| Cost (%CPU)| Time	   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	   |  6647K|   158M|	   | 67481   (1)| 00:13:30 |
|   1 |  HASH UNIQUE	   |	   |  6647K|   158M|   229M| 67481   (1)| 00:13:30 |
|   2 |   TABLE ACCESS FULL| TEST1 |  6647K|   158M|	   | 19570   (1)| 00:03:55 |
------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  4  recursive calls
	  0  db block gets
     142133  consistent gets
      71297  physical reads
	  0  redo size
       2749  bytes sent via SQL*Net to client
	496  bytes received via SQL*Net from client
	  9  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
	107  rows processed


SQL> select employee_id,first_name from test1 group by employee_id,first_name;

107 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1431305903

----------------------------------------------------------------------------
| Id  | Operation	   | Name  | Rows  | Bytes | Cost (%CPU)| Time	   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	   |  6647K|   158M| 19781   (2)| 00:03:58 |
|   1 |  HASH GROUP BY	   |	   |  6647K|   158M| 19781   (2)| 00:03:58 |
|   2 |   TABLE ACCESS FULL| TEST1 |  6647K|   158M| 19570   (1)| 00:03:55 |
----------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  4  recursive calls
	  0  db block gets
     142133  consistent gets
      71297  physical reads
	  0  redo size
       2749  bytes sent via SQL*Net to client
	496  bytes received via SQL*Net from client
	  9  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
	107  rows processed