深入理解Oracle索引(9)：不要过分依赖 HINT

   很多应用开发人员、特别是技术大拿、喜欢通过 HINT 强制Oracle CBO按自己的意志选择执行计划
   Rocky 窃以为、这种策略在某种程度上其实是把双刃剑
   使用 Hint 指定 Index 的时候需要慎重、如果不合适或者无效、可能画蛇添足、效率更低
   
   摘自官方文档：

If the INDEX hint specifies no indexes, then the optimizer considers the cost of a scan on each available index on the table and
then performs the index scan with the lowest cost. The database can also choose to scan multiple indexes and merge the results,
if such an access path has the lowest cost. The optimizer does not consider a full table scan.

   如果我们使用 HINT 指定了一个无效的 Index、CBO 会扫描表中所有可以使用的 Index
   然后选择 cost 最小的 Index 或者复合索引、而不会选择全表扫描
   
   测试：
   
   ① 创建一个表、含有 Bitmap Index 和 B-tree Index 

sys@ORCL> create table rocky as select object_name,object_id from dba_objects;

Table created.

sys@ORCL> create index b_tree_rocky on rocky (object_id);

Index created.

sys@ORCL> create bitmap index bitmap_rocky on rocky (object_name);

Index created.

sys@ORCL> exec dbms_stats.gather_table_stats(USER,'ROCKY',cascade=>true);

PL/SQL procedure successfully completed.

   ② 不使用 HINT

sys@ORCL> set autot trace exp stat
sys@ORCL> select object_id from rocky;

50319 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1770888108

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50319 | 245K| 55 (2)| 00:00:01 |
| 1 | TABLE ACCESS FULL| ROCKY | 50319 | 245K| 55 (2)| 00:00:01 |
---------------------------------------------------------------------------

Statistics
----------------------------------------------------------
138 recursive calls
0 db block gets
3603 consistent gets
0 physical reads
0 redo size
728242 bytes sent via SQL*Net to client
37279 bytes received via SQL*Net from client
3356 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
50319 rows processed

     这里因为 object_id　可能有 NULL 、所以不会使用 b_tree_rocky 索引、意料之中
     
   ③ 指定 B-tree Index 

sys@ORCL> select /*+ INDEX(rocky b_tree_rocky)*/object_id from rocky;

50319 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 105445584

---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50319 | 245K| 782 (1)| 00:00:10 |
| 1 | TABLE ACCESS BY INDEX ROWID | ROCKY | 50319 | 245K| 782 (1)| 00:00:10 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
| 3 | BITMAP INDEX FULL SCAN | BITMAP_ROCKY | | | | |
---------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
27546 consistent gets
0 physical reads
0 redo size
2145896 bytes sent via SQL*Net to client
37279 bytes received via SQL*Net from client
3356 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50319 rows processed

     因为 object_id 有 NULL、所以不会走 b_tree Index 
     但就算不会使用b_tree_rocky index也不应该会使用BITMAP_rocky index、毕竟使用这个的cost会大于全表扫描
     
   ④ 指定一个 无效的 Index 

sys@ORCL> select /*+ INDEX(rocky ooxx)*/object_id from rocky;

50319 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 105445584

---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50319 | 245K| 782 (1)| 00:00:10 |
| 1 | TABLE ACCESS BY INDEX ROWID | ROCKY | 50319 | 245K| 782 (1)| 00:00:10 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
| 3 | BITMAP INDEX FULL SCAN | BITMAP_ROCKY | | | | |
---------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
27546 consistent gets
0 physical reads
0 redo size
2145896 bytes sent via SQL*Net to client
37279 bytes received via SQL*Net from client
3356 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50319 rows processed

   因为我们 HINT 指定 b_tree_rocky index的时候、因为 object_id 可能有值为空(列没定义为not null)
   所以不能使用该 index、从而也就是相当于一个无效的 index、从而扫描该表的其他可以使用的index、导致使用了位图索引(该类型index不排除null)、而非全表扫描