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)
