Oracle全表扫描及其执行计划

　　5、小表的全表扫描是否高效？

--使用scott下dept表，仅有4行数据
scott@ORA11G> select * from dept where deptno>10;

3 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2985873453            --->执行计划选择了索引扫描
---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     3 |    60 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| DEPT    |     3 |    60 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | PK_DEPT |     3 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("DEPTNO">10)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets                      -->使用了4次逻辑读
          0  physical reads
          0  redo size
        515  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

-->下面强制使用全表扫描
scott@ORA11G> select /*+ full(dept) */ * from dept where deptno>10;

3 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3383998547
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     3 |    60 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| DEPT |     3 |    60 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("DEPTNO">10)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets         -->此时的逻辑读同样为4次
          0  physical reads
          0  redo size
        515  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

--下面来看看count(*)的情形
scott@ORA11G> select count(*) from dept;

1 row selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3051237957               --->执行计划选择了索引全扫描
--------------------------------------------------------------------
| Id  | Operation        | Name    | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT |         |     1 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |         |     1 |            |          |
|   2 |   INDEX FULL SCAN| PK_DEPT |     4 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          1  consistent gets            -->逻辑读仅为1次
          0  physical reads
          0  redo size
        335  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

-->下面强制使用全表扫描
scott@ORA11G> select /*+ full(dept) */ count(*) from dept;

1 row selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 315352865
-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets     -->使用了3次逻辑读
          0  physical reads
          0  redo size
        335  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
                  
--对于小表，从上面的情形可以看出，使用索引扫描也是比全表扫描高效
--因此，建议始终为小表建立索引