索引系列四 --索引特性之存列值优化count

--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