索引唯一性掃描(INDEX UNIQUE SCAN)

 

     索引唯一性掃描（INDEX UNIQUE SCAN）是針對唯一性索引（UNIQUE INDEX）的掃描，它僅僅適用於where條件裡是等值查詢的目標SQL。因為掃描的物件是唯一性索引，所以索引唯一性掃描的結果至多隻會返回一條記錄。

Emp表中empno為primary key，對應一個unique index

  
SCOTT@PDBORCL> select * from emp where empno=7369;  
執行計劃 
---------------------------------------------------------- 
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)  
統計資訊 
---------------------------------------------------------- 
         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> 

從上述顯示內容可以看出，"select * from emp where empno=7369"的執行計劃走的是索引唯一性掃描