INDEX FAST FULL SCAN和INDEX FULL SCAN

 INDEX FULL SCAN 索引全扫描。单块读 。它扫描的结果是有序的，因为索引是有序的。它通常发生在     

                                        下面几种情况(注意:即使SQL满足以下情况 不一定会走索引全扫描)

                                       1. SQL语句有order by选项，并且order by 的列都包含

                                        在索引中，并且order by 后列顺序必须和索引列顺序一致。

                                       2. 在进行SORT MERGE JOIN的时候，如果要查询的列通过索 

                                          引就能获得，那就不必进行全表扫描了，另外也避免了排

                                          序，因为INDEX FULL SCAN返回的结果已经排序。

                                       3. 当查询中有GROUP BY，并且GROUP BY 的列包含在索引中。

                               等待事件：db file sequential read

                               HINT: INDEX(表名/别名 索引名)

   INDEX FAST FULL SCAN 索引快速全扫描。多块读 。当SQL要查询的数据能够完全从索引中获得，那么

                                                 Oracle就不会走全表扫描了，就会走索引快速全

                                                 扫描。索引快速全扫描类似全表扫描，它可以多块 

                                                 读，并且可以并行扫描。

                               等待事件：db file scattered read

                               HINT：INDEX_FFS(表名/别名 索引名) 



测试INDEX FAST FULL SCAN:

create table test as select * from dba_objects;





create index test_idx1 on test(object_name);



BEGIN

  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'HR',

                                tabname          => 'TEST',

                                estimate_percent => 30,

                                method_opt       => 'for all columns size repeat',

                                no_invalidate    => FALSE,

                                degree           => 8,

                                cascade          => TRUE);

END;



SQL> explain plan for select object_name from test;



Explained.



SQL> select * from table(dbms_xplan.display());



PLAN_TABLE_OUTPUT

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



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

Plan hash value: 1357081020



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

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

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

|   0 | SELECT STATEMENT  |	 |  1159K|    27M|  4687   (1)| 00:00:57 |

|   1 |  TABLE ACCESS FULL| TEST |  1159K|    27M|  4687   (1)| 00:00:57 |

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



8 rows selected.





--从上面的执行计划中可知，此时走了全表扫描。   

--由于我们需要查询的列为object_name，因此理论上只需要读取索引就应该可以返回所有数据，而此时为什么是全表扫描呢？   

--这是因为NULL值与索引的特性所决定的。即null值不会被存储到B树索引。因此应该为表 test 的列 object_name 添加 not null 约束,或者

where条件里加上is not null



此例中虽然SQL> select count(*) from test where object_name is null;



  COUNT(*)

----------

	 0

object_name 不存在空值-------------------





 SQL> set autot trace exp

SQL>  select object_name from test where object_name is not null;



Execution Plan

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

Plan hash value: 703934364



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

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

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

|   0 | SELECT STATEMENT     |		 |  1161K|    27M|  1563   (1)| 00:00:19 |

|*  1 |  INDEX FAST FULL SCAN| TEST_IDX1 |  1161K|    27M|  1563   (1)| 00:00:19 |

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



Predicate Information (identified by operation id):

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



   1 - filter("OBJECT_NAME" IS NOT NULL)





---INDEX FULL SCAM

SQL>  select object_name from test order by object_name;



Execution Plan

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

Plan hash value: 2007178810



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

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

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

|   0 | SELECT STATEMENT   |	  |  1161K|    27M|	  | 13060   (1)| 00:02:37 |

|   1 |  SORT ORDER BY	   |	  |  1161K|    27M|    35M| 13060   (1)| 00:02:37 |

|   2 |   TABLE ACCESS FULL| TEST |  1161K|    27M|	  |  4687   (1)| 00:00:57 |

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



SQL> select object_name from test where object_name is not null order by object_name;



Execution Plan

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

Plan hash value: 436181854



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

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

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

|   0 | SELECT STATEMENT |	     |	1161K|	  27M|	5754   (1)| 00:01:10 |

|*  1 |  INDEX FULL SCAN | TEST_IDX1 |	1161K|	  27M|	5754   (1)| 00:01:10 |

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



Predicate Information (identified by operation id):

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



   1 - filter("OBJECT_NAME" IS NOT NULL)