Oracle表链接方法有四种:
● 排序合并链接(Sort Merge Join)
● 嵌套循环链接(Nested Loops Join)
● 哈希链接(Hash Join)
● 笛卡尔积(Cartesian Product) oop
嵌套循环链接是Oracle将链接的两个表根据结果集的大小,决定出驱动表和被驱动表。结果集小的做为驱动表,结果集大的做为被驱动表。对于驱动表的每一行,都要与被驱动表的全部行使用链接条件进行匹配。适用于驱动表结果集很小,被驱动表在链接列上有高效索引的表链接。能够用于全部链接条件。对应的执行计划为NESTED LOOPS。 spa
SQL> select e.employee_id,e.last_name,d.department_name from employees e,departments d where e.department_id=d.department_id and d.department_id=80; .net
34 rows selected. blog
Execution Plan
----------------------------------------------------------
Plan hash value: 1492013603 排序
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 34 | 1054 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 34 | 1054 | 4 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 1 | 16 | 1 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | DEPT_ID_PK | 1 | | 0 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL | EMPLOYEES | 34 | 510 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------- 索引
这里DEPARTMENTS表加了条件过滤后结果集只有1行,因此被选为驱动表。 博客
SQL> select e.first_name,e.last_name,e.salary,d.department_name from employees e,departments d where d.department_name IN ('Marketing', 'Sales') and e.department_id = d.department_id; hash
36 rows selected. it
Execution Plan
----------------------------------------------------------
Plan hash value: 1021246405 io
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 19 | 722 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 19 | 722 | 4 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 20 | 722 | 4 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL | DEPARTMENTS | 2 | 32 | 3 (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 |
--------------------------------------------------------------------------------------------------
这里有2次NESTED LOOPS,是由于Oracle 11g引入了引入了向量I/O(Vector I/O),批量处理多个物理I/O请求来提升嵌套循环链接的效率。
若是驱动表的限制条件的字段上有索引,被驱动表的链接条件的字段上有索引,则使用嵌套循环链接的效率就会很高。
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转载于:http://blog.itpub.net/28536251/viewspace-2140231/