索引唯一性扫描(INDEX UNIQUE SCAN)

 

     索引唯一性扫描（INDEX UNIQUE SCAN）是针对唯一性索引（UNIQUE INDEX）的扫描，它仅仅适用于where条件里是等值查询的目标SQL。因为扫描的对象是唯一性索引，所以索引唯一性扫描的结果至多只会返回一条记录。

Emp表中empno为primary key，对应一个unique index

            
SCOTT@PDBORCL> select * from emp where empno=7369;             
执行计划        
----------------------------------------------------------        
Plan hash value: 2949544139      --------------------------------------------------------------------------------------       
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |        
--------------------------------------------------------------------------------------        
|   0 | SELECT STATEMENT            |        |     1 |    38 |     1   (0)| 00:00:01 |        
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    38 |     1   (0)| 00:00:01 |        
|*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |        
--------------------------------------------------------------------------------------      Predicate Information (identified by operation id):       
---------------------------------------------------         2 - access("EMPNO"=7369)             
统计信息        
----------------------------------------------------------        
         69  recursive calls        
          0  db block gets        
         95  consistent gets        
         21  physical reads        
          0  redo size        
        898  bytes sent via SQL*Net to client        
        533  bytes received via SQL*Net from client        
          1  SQL*Net roundtrips to/from client        
          8  sorts (memory)        
          0  sorts (disk)        
          1  rows processed      SCOTT@PDBORCL>   

从上述显示内容可以看出，"select * from emp where empno=7369"的执行计划走的是索引唯一性扫描