解读nestloop和hash join

最新推荐文章于 2024-04-03 17:03:00 发布

原创最新推荐文章于 2024-04-03 17:03:00 发布 · 1k 阅读

1 ·

CC 4.0 BY-SA版权

本文展示了一个具体的SQL查询案例，通过嵌套循环连接两个表并应用过滤条件，最终返回符合条件的数据记录。同时，展示了如何使用Oracle的dbms_xplan工具来查看SQL执行计划，包括所采用的操作类型、预计与实际行数等详细信息。

SQL> set lines 200
SQL> set pages 200
SQL> alter session set statistics_level = all;

NESTLOOP：嵌套循环

SQL> select e.ename,e.job,d.dname from emp e, dept d where
2 e.deptno=d.deptno and e.sal < 2000;

ENAME      JOB       DNAME
---------- --------- --------------
SMITH      CLERK     RESEARCH
ALLEN      SALESMAN SALES
WARD       SALESMAN SALES
MARTIN     SALESMAN SALES
TURNER     SALESMAN SALES
ADAMS      CLERK     RESEARCH
JAMES      CLERK     SALES
MILLER     CLERK     ACCOUNTING

已选择8行。

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------------

SQL_ID anvuxahhpxp8g, child number 0
-------------------------------------
select e.ename,e.job,d.dname from emp e, dept d where e.deptno=d.deptno and e.sal < 2000

Plan hash value: 351108634

--------------------------------------------------------------------------------------------------
| Id | Operation                    | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
--------------------------------------------------------------------------------------------------
|   1 | NESTED LOOPS                |         |      1 |      4 |      8 |00:00:00.01 |      18 |
|* 2 |   TABLE ACCESS FULL          | EMP     |      1 |      4 |      8 |00:00:00.01 |       8 |
|   3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |      8 |      1 |      8 |00:00:00.01 |      10 |
|* 4 |    INDEX UNIQUE SCAN         | PK_DEPT |      8 |      1 |      8 |00:00:00.01 |       2 |
--------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("E"."SAL"<2000)
4 - access("E"."DEPTNO"="D"."DEPTNO")

已选择21行。