从11g开始, Nested Loop Joins 的i/o操作有了优化, 叫做vector i/o
Oracle Database 11g introduces a new implementation for nested loop joins to reduce overall latency for physical I/O. When an index or a table block is not in the buffer cache and is needed to process the join, a physical I/O is required. Oracle Database 11g can batch multiple physical I/O requests and process them using a vector I/O instead of processing them one at a time.

执行计划出现了两次"NESTED LOOPS"
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| Id | Operation | Name | Rows | Bytes | Cost(%CPU)| Time |
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| 0 | SELECT STATEMENT | | 19 | 722 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 19 | 722 | 3 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL | DEPARTMENTS | 2 | 32 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 10 | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| EMPLOYEES | 10 | 220 | 1 (0)| 00:00:01 |
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Predicate Information (identified by operation id):
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3 - filter("D"."DEPARTMENT_NAME"='Marketing' OR "D"."DEPARTMENT_NAME"='Sales')
4 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

12C开始出现了BATCHED TABLE ACCESS，
The BATCHED access means that the database retrieves a few rowids from
the index, and then attempts to access rows in block order to improve the clustering
and reduce the number of times that the database must access a block.

执行计划：
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|Id| Operation | Name |Rows|Bytes|Cost(%CPU)|Time|
--------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | | |2(100)| |
| 1| TABLE ACCESS BY INDEX ROWID BATCHED|EMPLOYEES |16|1104|2 (0)|00:00:01|
|*2| INDEX RANGE SCAN |EMP_EMP_ID_PK|16| |1 (0)|00:00:01|
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Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPLOYEE_ID">190)

ss

vector i/o and BATCHED TABLE ACCESS