索引特性之存列值优化sum avg

--要领：只要索引能回答问题，索引就可以当成一个"瘦表"，访问路径就会减少。另外切记不存储空值

drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create index idx1_object_id on t(object_id);
set autotrace on
select count(*) from t;
执行计划
-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   292   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 69485 |   292   (1)| 00:00:04 |
-------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1048  consistent gets

--为啥用不到索引，因为索引不能存储空值，所以加上一个is not null,再试验看看            
select count(*) from t where object_id is not null;
执行计划
----------------------------------------------------------------------------------------
| Id  | Operation             | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                |     1 |    13 |    50   (2)| 00:00:01 |
|   1 |  SORT AGGREGATE       |                |     1 |    13 |            |          |
|*  2 |   INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 69485 |   882K|    50   (2)| 00:00:01 |
----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        170  consistent gets

--也可以不加is not null，直接把列的属性设置为not null，也成，继续试验如下：
alter table t modify OBJECT_ID not null;
select count(*) from t;
执行计划
--------------------------------------------------------------------------------
| Id  | Operation             | Name           | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                |     1 |    49   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |                |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 69485 |    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
        425  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

--如果是主键就无需定义列是否允许为空了。
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
alter table t add constraint pk1_object_id primary key (OBJECT_ID);
set autotrace on
select count(*) from t;

执行计划
-------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               |     1 |    46   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |               |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| PK1_OBJECT_ID | 69485 |    46   (0)| 00:00:01 |
-------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        160  consistent gets