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