SUM/AVG的优化
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。