Oracle 索引扫描的几种情况

index range scan(索引范围扫描)：

1.对于unique index来说，如果where 条件后面出现了<,> ,between ...and...的时候，那么就可能执行index range scan,如果where条件后面是=，那么就会执行index unique scan。

2.对于none unique index来说 如果where 条件后面出现了=,>,<,betweed...and...的时候，就有可能执行index range scan。

3.对于组合索引来说，如果where条件后面出现了组合索引的引导列，那么可能执行index range scan。

index fast full scan(索引快速全扫描)：

如果select 语句后面中的列都被包含在组合索引中，而且where后面没有出现组合索引的引导列，并且需要检索出大部分数据，那么这个时候可能执行index fast full scan。index fast full scan 发生的条件:

1.必须是组合索引   ？。2.引导列不在where条件中

index skip scan(索引跳跃式扫描)

当查询可以通过组合索引得到结果，而且返回结果很少，并且where条件中没有包含索引引导列的时候，可能执行index skip scan

索引跳跃式扫描发生的条件:

1.必须是组合索引。

2.引导列没有出现在where条件中

-eg1

SQL> create table test as select * from dba_objects;

Table created.

SQL> create unique index ind_id on test(object_id);

Index created.

SQL> create index ind_owner on test(owner);

Index created.

SQL> create index ooo on test(owner,object_name,object_type);

Index created.

SQL> exec dbms_stats.gather_table_stats('SCOTT','TEST');

PL/SQL procedure successfully completed.

SQL> set autot trace

SQL> select owner from test where object_id=10;

Execution Plan
----------------------------------------------------------
Plan hash value: 2544773305

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

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

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

|   0 | SELECT STATEMENT        |         |       1 |      11 |       2   (0)| 00:0
0:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST   |       1 |      11 |       2   (0)| 00:0
0:01 |

|*  2 |   INDEX UNIQUE SCAN        | IND_ID |       1 |         |       1   (0)| 00:0
0:01 |

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

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=10)

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

SQL> select owner from test where object_id<10;

8 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1361604213

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

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

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

|   0 | SELECT STATEMENT        |         |       8 |      88 |       3   (0)| 00:0
0:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST   |       8 |      88 |       3   (0)| 00:0
0:01 |

|*  2 |   INDEX RANGE SCAN        | IND_ID |       8 |         |       2   (0)| 00:0
0:01 |

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

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"<10)

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      5  consistent gets
      0  physical reads
      0  redo size
    609  bytes sent via SQL*Net to client
    524  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      8  rows processed

对于唯一索引，发生index range scan的时候就是返回多行记录，where后面有<,>,between..and 等返回扫描

SQL> select owner from test where owner='SCOTT';

Execution Plan
----------------------------------------------------------
Plan hash value: 2280863269

------------------------------------------------------------------------------
| Id  | Operation     | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |         |    3613 | 21678 |       9   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IND_OWNER |    3613 | 21678 |       9   (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("OWNER"='SCOTT')

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      3  consistent gets
      0  physical reads
      0  redo size
    526  bytes sent via SQL*Net to client
    524  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed

对于非唯一索引，即使where后面是=条件，但也可能返回多行，也是index range scan扫描

SQL> select object_name,object_type from test where owner='SCOTT';

Execution Plan
----------------------------------------------------------
Plan hash value: 2845720098

-------------------------------------------------------------------------
| Id  | Operation     | Name | Rows    | Bytes | Cost (%CPU)| Time    |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |    |  3613 |   141K|    28   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| OOO    |  3613 |   141K|    28   (0)| 00:00:01 |
-------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("OWNER"='SCOTT')

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      4  consistent gets
      0  physical reads
      0  redo size
    610  bytes sent via SQL*Net to client
    524  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed

因为这个索引不是唯一索引，where后面的列用到了索引000，所以进行index range scan

SQL> select owner, object_name,object_type from test where object_name='EMP' ;

no rows selected

Execution Plan
----------------------------------------------------------
Plan hash value: 1799988433

-------------------------------------------------------------------------
| Id  | Operation     | Name | Rows    | Bytes | Cost (%CPU)| Time    |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |    |     2 |    80 |    26   (0)| 00:00:01 |
|*  1 |  INDEX SKIP SCAN | OOO    |     2 |    80 |    26   (0)| 00:00:01 |
-------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("OBJECT_NAME"='EMP')
       filter("OBJECT_NAME"='EMP')

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

上面的查询可以通过索引000来得到，并且where后面没有用到索引列，而且返回的行数很少（。）所以cbo选择index skip scan

select owner, object_name,object_type from test where object_type='INDEX';
Execution Plan
----------------------------------------------------------
Plan hash value: 3464522019

-----------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |        |  1971 | 78840 |    168   (1)| 00:00:03 |
|*  1 |  INDEX FAST FULL SCAN| OOO  |  1971 | 78840 |    168   (1)| 00:00:03 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("OBJECT_TYPE"='INDEX')

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
    957  consistent gets
      0  physical reads
      0  redo size
     199834  bytes sent via SQL*Net to client
       4253  bytes received via SQL*Net from client
    341  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
       5088  rows processed

同上，但是这里返回行数较多，cbo选择了index fast full scan，避免了全表扫描