【索引】索引五种扫描方式至索引范围扫描

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索引范围扫描，根据字面理解就是在索引叶子节点链表上，通过一定的取值范围来扫描。由于索引是排序的，所以当按照范围来扫描时，效率也是极高的。

SQL> select empno,ename from emp where empno >= 7369 and empno <= 7800;

EMPNO ENAME
---------- ----------
7369 SMITH
7499 ALLEN
7521 WARD
7566 JONES
7654 MARTIN
7698 BLAKE
7782 CLARK
7788 SCOTT

已选择8行。

执行计划
----------------------------------------------------------
Plan hash value: 169057108

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 120 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 12 | 120 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | PK_EMP | 12 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

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

2 - access("EMPNO">=7369 AND "EMPNO"<=7800)

统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
621 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed

SQL>

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