sort group by和hash group by性能比较

        在10gR2中，group by由以前的sort group by改成了hash group by，这种算法上的改进，取消了sort group by必须进行的排序操作。官方文档上说hash group by的性能强于sort group by，但经过我的测试，不管是逻辑读和cost都是一样的。

SQL>create table test as select * from dba_objects; 

SQL> set timing on
SQL> set autotrace traceonly
SQL> select status,count(*) from test group by status;
已用时间:  00: 00: 00.03
执行计划
----------------------------------------------------------
Plan hash value: 1435881708
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     2 |    14 |   286   (4)| 00:00:04 |
|   1 |  HASH GROUP BY     |      |     2 |    14 |   286   (4)| 00:00:04 |
|   2 |   TABLE ACCESS FULL| TEST | 89088 |   609K|   279   (2)| 00:00:04 |
---------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1230  consistent gets
          0  physical reads
          0  redo size
        432  bytes sent via SQL*Net to client
        350  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          2  rows processed
         
SQL> alter session set "_gby_hash_aggregation_enabled" = false;
会话已更改。

SQL> select status,count(*) from test group by status;
已用时间:  00: 00: 00.04
执行计划
----------------------------------------------------------
Plan hash value: 2603667166

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     2 |    14 |   286   (4)| 00:00:04 |
|   1 |  SORT GROUP BY     |      |     2 |    14 |   286   (4)| 00:00:04 |
|   2 |   TABLE ACCESS FULL| TEST | 89088 |   609K|   279   (2)| 00:00:04 |
---------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1230  consistent gets
          0  physical reads
          0  redo size
        432  bytes sent via SQL*Net to client
        350  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          2  rows processed