一句Delete..In.. 删除语句的优化

实验环境：

#	类别	版本
1	操作系统	Win10
2	数据库	Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
3	硬件环境	T440p
4	内存	8G

建表：

CREATE TABLE tb_sc
(
    id NUMBER not null primary key,
    studentid int not null,
    courseid int not null,
    score int not null
)

充值：

Insert into tb_sc
select rownum,dbms_random.value(0,10000),dbms_random.value(1,5),dbms_random.value(0,150) from dual
connect by level<=10000
order by dbms_random.random

待优化的SQL，此sql在数据量大时如僵死一般：

delete from tb_sc where (studentid,courseid,score) not in (select studentid,courseid,max(score) as score from tb_sc group by studentid,courseid);

优化方案1：

delete from tb_sc where id not in (select tb_sc.id from tb_sc,( select studentid,courseid,max(score) as score from tb_sc group by studentid,courseid ) tb_sc2
where tb_sc.studentid=tb_sc2.studentid and tb_sc.courseid=tb_sc2.courseid and tb_sc.score=tb_sc2.score)

优化方案2：

delete from tb_sc where not exists (
select null from tb_sc a,
                 (select studentid,courseid,max(score) as score from tb_sc group by studentid,courseid) b
where a.studentid=b.studentid and a.courseid=b.courseid and a.score=b.score and tb_sc.id=a.id)

优化方案3: 这种方案适用于delete语句太简单而删除数据较多的场合：

每次删除一百条：

delete from tb_sc where (studentid,courseid,score) not in (select studentid,courseid,max(score) as score from tb_sc group by studentid,courseid) and rownum<101

放在循环里执行：

while(剩余数量>0){

          删除符合条件的

}

优化方案四：将要保留的数据存入一张临时表，删除原表再倒回来，这种操作最大的优势在于降低表的水位线。

相关帖子:

https://www.cnblogs.com/xiandedanteng/p/12232822.html

https://www.cnblogs.com/xiandedanteng/p/12232485.html

https://www.cnblogs.com/xiandedanteng/p/12231995.html

--2020年2月5日--