索引范围扫描(INDEX RANGE SCAN)
索引范围扫描（INDEX RANGE SCAN）适用于所有类型的B树索引，当扫描的对象是唯一性索引时，此时目标SQL的where条件一定是范围查询（谓词条件为 BETWEEN、<、>等）；当扫描的对象是非唯一性索引时，对目标SQL的where条件没有限制（可以是等值查询，也可以是范围查询）。 索引范围扫描的结果可能会返回多条记录，其实这就是索引范围扫描中"范围"二字的本质含义。
测试一:唯一索引的范围查询
SCOTT@PDBORCL> select * from emp where empno>7369; 已选择 13 行。 执行计划 ---------------------------------------------------------- Plan hash value: 2787773736 ---------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 13 | 494 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP | 13 | 494 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | PK_EMP | 13 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO">7369) 统计信息 ---------------------------------------------------------- 54 recursive calls 0 db block gets 97 consistent gets 21 physical reads 0 redo size 1607 bytes sent via SQL*Net to client 544 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 8 sorts (memory) 0 sorts (disk) 13 rows processed SCOTT@PDBORCL>
SCOTT@PDBORCL> select * from emp where empno>7369 order by empno desc; (降序排列)
SCOTT@PDBORCL> select * from emp where empno>7369 order by empno desc; 已选择 13 行。 执行计划 ---------------------------------------------------------- Plan hash value: 2707196548 --------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 13 | 494 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID | EMP | 13 | 494 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN DESCENDING| PK_EMP | 13 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO">7369) 统计信息 ---------------------------------------------------------- 54 recursive calls 0 db block gets 97 consistent gets 21 physical reads 0 redo size 1607 bytes sent via SQL*Net to client 544 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 8 sorts (memory) 0 sorts (disk) 13 rows processed SCOTT@PDBORCL>
这个执行计划和上面的执行计划有什么区别呢？其中第一个执行计划是索引范围扫描（INDEX RANGE SCAN），但是第二个执行计划是索引范围降序扫描（INDEX RANGE SCAN DESCENDING）。由此可以看出对于唯一索引，ORACLE是已经进行了正序排列了。也就是唯一索引在进行排序的时候消耗不是很大的，因为它在保存 的时候就按照升序进行保存的。
测试二:非唯一索引的等值查询
创建一个测试表EMP_TEMP：
create table emp_temp as select * from emp;
然后在表EMP_TEMP的列EMPNO上创建一个单键值非唯一性同名B树索引IDX_EMP_TEMP：
create index idx_emp_temp on emp_temp(empno);
最后我们再次执行如下SQL：
select * from emp_temp where empno=7369;
执行计划如下:
SCOTT@PDBORCL> create table emp_temp as select * from emp; 表已创建。 SCOTT@PDBORCL> create index idx_emp_temp on emp_temp(empno); 索引已创建。 SCOTT@PDBORCL> select * from emp_temp where empno=7369; 执行计划 ---------------------------------------------------------- Plan hash value: 1638992559 ---------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP_TEMP | 1 | 38 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_EMP_TEMP | 1 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO"=7369) 统计信息 ---------------------------------------------------------- 46 recursive calls 0 db block gets 76 consistent gets 16 physical reads 0 redo size 1042 bytes sent via SQL*Net to client 544 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 6 sorts (memory) 0 sorts (disk) 1 rows processed SCOTT@PDBORCL>
此SQL的执行计划已经从之前的索引唯一性扫描变为现在的索引范围扫描
测试三:非唯一索引的范围查询
同样是测试二中的表EMP_TEMP
SCOTT@PDBORCL> select * from emp_temp where empno>7369;
执行计划如下:
SCOTT@PDBORCL> select * from emp_temp where empno>7369; 已选择 13 行。 执行计划 ---------------------------------------------------------- Plan hash value: 1638992559 ---------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 13 | 494 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP_TEMP | 13 | 494 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_EMP_TEMP | 13 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO">7369) 统计信息 ---------------------------------------------------------- 46 recursive calls 0 db block gets 77 consistent gets 15 physical reads 0 redo size 1607 bytes sent via SQL*Net to client 544 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 6 sorts (memory) 0 sorts (disk) 13 rows processed SCOTT@PDBORCL>
