关于group by

<pre name="code" class="sql">关于group by 排序问题

10g 以前sort group by  需要排序

10g 以后 hash group by 不需要排序
SQL> set linesize 200
SQL> set pagesize 200
SQL> set autot trace
SQL> select owner,count(*) from t1 group by owner;

已选择30行。


执行计划
----------------------------------------------------------
Plan hash value: 136660032

---------------------------------------------------------------------------
| Id  | Operation	   | Name | Rows  | Bytes | Cost (%CPU)| Time	  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |    30 |   180 |  2014   (2)| 00:00:25 |
|   1 |  HASH GROUP BY	   |	  |    30 |   180 |  2014   (2)| 00:00:25 |
|   2 |   TABLE ACCESS FULL| T1   |   580K|  3403K|  1998   (1)| 00:00:24 |
---------------------------------------------------------------------------


统计信息
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
       8323  consistent gets
       8313  physical reads
	  0  redo size
       1059  bytes sent via SQL*Net to client
	426  bytes received via SQL*Net from client
	  3  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
	 30  rows processed

转载于:https://www.cnblogs.com/zhaoyangjian724/p/3797855.html