INDEX UNIQUE SCAN 索引唯一扫描。单块读 只可能发生在unique index/primary key 等值查找
等待事件：db file sequential read 但是你几乎看不到，因为只读一条数据
HINT 无需指定，有索引会自动走INDEX UNIQUE SCAN
唯一索引等值扫描，需要几个I/O？
需要索引高度个IO
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
