ORACLE SQL效率 实践

重点关注9:用EXISTS替代IN.重点关注

案例 ：查找DEPT_1部门的人员姓名

9.1 SQL语句

--子查询

select pname from t_person where deptid in (

       select deptid from t_department where deptname like 'DEPT_1'

);

--exsist

select pname from t_person where exists (

       select * from t_department where t_department.deptid = t_person.deptid

       and deptname like 'DEPT_1'

);

--外连接
select pname
  from t_person p LEFT join t_department d ON p.deptid = d.deptid
 WHERE d.deptname like 'DEPT_1';

9.2 测试代码块

--子查询代码块测试

  declare

        v_sql  varchar2(300);

        v_diff number;

      begin

        v_sql := 'select pname from t_person where deptid in (

         select deptid from t_department where deptname like ''DEPT_1'') ';

        select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

      end;

  

--exists代码块测试

    declare

      v_sql  varchar2(300);

      v_diff number;

    begin

      v_sql := 'select pname from t_person where exists (

       select * from t_department where t_department.deptid = t_person.deptid

       and deptname like ''DEPT_1'')';

      select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

    end;

 

--外连接代码块测试

    declare

      v_sql  varchar2(300);

      v_diff number;

    begin

      v_sql := 'select pname

  from t_person p LEFT join t_department d ON p.deptid = d.deptid

 WHERE d.deptname like ''DEPT_1''';

      select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

    end;

9.3 输出结果

Time differences is  10



Time differences is  9



Time differences is  10

9.4

效率由高到低：exists> 外连接=子查询

重点关注10:用NOT EXISTS替代NOT IN .

案例 ：查找 非DEPT_1部门的 人员姓名

10.1 SQL语句

 --子查询

 SELECT  p.pname

FROM T_PERSON

WHERE DEPTID NOT IN (SELECT DEPTID

FROM T_DEPARTMENT

WHERE DEPTNAME LIKE 'DEPT_1');

 --外连接 查询结果包含没有部门的人员

 select p.pname

  from t_person p left join t_department d

 on p.deptid = d.deptid 

   where d.DEPTNAME  not like 'DEPT_1' or d.DEPTNAME is null;

--exists法  查询结果不包含没有部门的人员

select p.pname

  from t_person p

 where exists (

 select * from t_department  d where  p.deptid = d.deptid and d.DEPTNAME  not like 'DEPT_1'  );

10.2 执行代码块(在每个测试之前都要清空数据库缓存)

 

--子查询代码块测试

declare

      v_sql  varchar2(300);

      v_diff number;

    begin

      v_sql := 'SELECT T_PERSON.pname

    FROM T_PERSON

    WHERE DEPTID NOT IN (SELECT DEPTID

    FROM T_DEPARTMENT

    WHERE DEPTNAME LIKE ''DEPT_1'')';

      select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

    end;

  

--外连接代码块测试

    declare

      v_sql  varchar2(300);

      v_diff number;

    begin

      v_sql := 'select p.pname

      from t_person p left join t_department d

     on p.deptid = d.deptid 

       where d.DEPTNAME  not like ''DEPT_1'' or d.DEPTNAME is null';

      select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

    end;

 

--exists代码块测试

    declare

      v_sql  varchar2(300);

      v_diff number;

    begin

      v_sql := 'select p.pname

      from t_person p

     where exists (

     select * from t_department  d where  p.deptid = d.deptid and d.DEPTNAME  not like ''DEPT_1'')';

      select F_TEST_TIME_efficiency(v_sql) into v_diff from dual;

    end;

 

10.3 清空oracle缓存的语句

ALTER SYSTEM FLUSH BUFFER_CACHE;  

ALTER SYSTEM FLUSH SHARED_POOL ;

ALTER SYSTEM FLUSH GLOBAL CONTEXT;

ALTER SYSTEM SET EVENTS='IMMEDIATE TRACE NAME FLUSH_CACHE';

 

10.4 输出结果

Time differences is  9

Time differences is  8

Time differences is  8

10.5 结论

效率由高到低：外连接=exists >子查询