SUM/AVG的优化

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drop table t purge;

create table t as select * from dba_objects;

create index idx1_object_id on t(object_id);

set autotrace on

set linesize 1000

set timing on 

select sum(object_id) from t; 

执行计划

----------------------------------------------------------------------------------------

| Id  | Operation             | Name           | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT      |                |     1 |    13 |    49   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE       |                |     1 |    13 |            |          |

|   2 |   INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 |  1173K|    49   (0)| 00:00:01 |

----------------------------------------------------------------------------------------

统计信息

----------------------------------------------------------

          0  recursive calls

          0  db block gets

        170  consistent gets

          0  physical reads

          0  redo size

        432  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

          

--比较一下假如不走索引的代价，体会一下这个索引的重要性

select /*+full(t)*/ sum(object_id) from t;  

SUM(OBJECT_ID)

--------------

  2732093100         

执行计划

---------------------------------------------------------------------------

| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |

---------------------------------------------------------------------------

|   0 | SELECT STATEMENT   |      |     1 |    13 |   292   (1)| 00:00:04 |

|   1 |  SORT AGGREGATE    |      |     1 |    13 |            |          |

|   2 |   TABLE ACCESS FULL| T    | 92407 |  1173K|   292   (1)| 00:00:04 |

---------------------------------------------------------------------------

统计信息

----------------------------------------------------------

          0  recursive calls

          0  db block gets

       1047  consistent gets

          0  physical reads

          0  redo size

        432  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed     

          

--起来类似的比如AVG，和SUM是一样的，如下：

select avg(object_id) from t; 

AVG(OBJECT_ID)

--------------

  37365.5338

执行计划

----------------------------------------------------------------------------------------

| Id  | Operation             | Name           | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT      |                |     1 |    13 |    49   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE       |                |     1 |    13 |            |          |

|   2 |   INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 |  1173K|    49   (0)| 00:00:01 |

----------------------------------------------------------------------------------------

统计信息

----------------------------------------------------------

          0  recursive calls

          0  db block gets

        170  consistent gets

          0  physical reads

          0  redo size

        448  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

--不知大家注意到没，这里的试验已经告诉我们了，OBJECT_ID列是否为空，也不影响SUM/AVG等聚合的结果。          

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