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  • 标量子查询真不如表联结吗?

    在数据库日新月异的时代,不实际跑跑,谁快谁慢还真不好说。

    有这么一个员工表:

    create table tb_emp03(
         id number(8,0) primary key,
         name nvarchar2(60),
         salary number(5,0),
         deptno number(2,0)
    )

    可以这么填充数据:

    Insert into tb_emp03
    select rownum,dbms_random.string('*',dbms_random.value(6,20)),dbms_random.value(1000,50000),dbms_random.value(0,10) from dual
    connect by level<=2000000
    order by dbms_random.random

    还有这么一个部门表:

    create table tb_dept03(
        id number(5,0) primary key,
        name nvarchar2(20)
    )

    它的数据是定死的:

    insert into tb_dept03(id,name) values('1','sales');
    insert into tb_dept03(id,name) values('2','manage');
    insert into tb_dept03(id,name) values('3','postsale');
    insert into tb_dept03(id,name) values('4','dev');
    insert into tb_dept03(id,name) values('5','manu');
    insert into tb_dept03(id,name) values('6','maintenance');
    insert into tb_dept03(id,name) values('7','fix');
    insert into tb_dept03(id,name) values('8','counter');
    insert into tb_dept03(id,name) values('9','sweeper');
    insert into tb_dept03(id,name) values('10','cleaner');

    然后,做个带标量子查询的sql,目的是把员工的部门名称秀出来:

    select emp.id,emp.name,emp.salary,emp.deptno,(select dept.name from tb_dept03 dept where dept.id=emp.deptno) as deptname from tb_emp03 emp where emp.salary>49990
    已选择381行。
    
    已用时间:  00: 00: 00.40

    用时0.4秒。

    而把上面的SQL改成左联结再跑:

    select emp.id,emp.name,emp.salary,emp.deptno,dept.name from tb_emp03 emp left join tb_dept03 dept on emp.deptno=dept.id where emp.salary>49990
    
    
    已选择381行。
    
    已用时间:  00: 00: 00.40

    发现运行时间两者几乎一样啊。

    再跑跑两者的解释计划:

    SQL> select emp.id,emp.name,emp.salary,emp.deptno,(select dept.name from tb_dept03 dept where dept.id=emp.deptno) as deptname from tb_emp03 emp where emp.salary>49990;
    已用时间:  00: 00: 00.00
    
    执行计划
    ----------------------------------------------------------
    Plan hash value: 84985261
    
    --------------------------------------------------------------------------------
    ------------
    
    | Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)
    | Time     |
    
    --------------------------------------------------------------------------------
    ------------
    
    |   0 | SELECT STATEMENT            |              |   418 | 42218 |  3609   (2)
    | 00:00:44 |
    
    |   1 |  TABLE ACCESS BY INDEX ROWID| TB_DEPT03    |     1 |    35 |     1   (0)
    | 00:00:01 |
    
    |*  2 |   INDEX UNIQUE SCAN         | SYS_C0011427 |     1 |       |     1   (0)
    | 00:00:01 |
    
    |*  3 |  TABLE ACCESS FULL          | TB_EMP03     |   418 | 42218 |  3609   (2)
    | 00:00:44 |
    
    --------------------------------------------------------------------------------
    ------------
    
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    
       2 - access("DEPT"."ID"=:B1)
       3 - filter("EMP"."SALARY">49990)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    SQL>

    Cost是3609

    再看看左联结的:

    SQL> select emp.id,emp.name,emp.salary,emp.deptno,dept.name from tb_emp03 emp left join tb_dept03 dept on emp.deptno=dept.id where emp.salary>49990;
    已用时间:  00: 00: 00.00
    
    执行计划
    ----------------------------------------------------------
    Plan hash value: 3854866026
    
    --------------------------------------------------------------------------------
    ---
    
    | Id  | Operation             | Name      | Rows  | Bytes | Cost (%CPU)| Time
      |
    
    --------------------------------------------------------------------------------
    ---
    
    |   0 | SELECT STATEMENT      |           |   418 | 56848 |  3612   (2)| 00:00:4
    4 |
    
    |*  1 |  HASH JOIN RIGHT OUTER|           |   418 | 56848 |  3612   (2)| 00:00:4
    4 |
    
    |   2 |   TABLE ACCESS FULL   | TB_DEPT03 |    10 |   350 |     3   (0)| 00:00:0
    1 |
    
    |*  3 |   TABLE ACCESS FULL   | TB_EMP03  |   418 | 42218 |  3609   (2)| 00:00:4
    4 |
    
    --------------------------------------------------------------------------------
    ---
    
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    
       1 - access("EMP"."DEPTNO"="DEPT"."ID"(+))
       3 - filter("EMP"."SALARY">49990)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    SQL>

    Cost是3612,和标量子查询也差不多。

    到这里,可以得出结论了,标量子查询不是肯定慢的,左联结也未必快了。

     在DB优化器越做越好的今天,很多以前的理念都不能墨守成规了。

    --2020-01-27--

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  • 原文地址:https://www.cnblogs.com/heyang78/p/12236920.html
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