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
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