一句Delete In 刪除語句的優化

實驗環境：#類別

版本1作業系統

win10

2資料庫

oracle database 11g enterprise edition release 11.2.0.1.0 - 64bit production

3硬體環境

t440p4記憶體

8g建表：

create

table
tb_sc
( id 
number
notnull
primary
key,
studentid 
intnot
null
, courseid 
intnot
null
, score 
intnot
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 
bylevel
<=
10000
order
by dbms_random.random

待優化的sql，此sql在資料量大時如僵死一般：

delete

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

優化方案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
bystudentid,courseid ) tb_sc2
where tb_sc.studentid=tb_sc2.studentid and tb_sc.courseid=tb_sc2.courseid and tb_sc.score=tb_sc2.score)

優化方案2：

delete

from tb_sc where
notexists
(select
null
from
tb_sc a,
(select studentid,courseid,max(score) as score from tb_sc group
bystudentid,courseid) b
where a.studentid=b.studentid and a.courseid=b.courseid and a.score=b.score and tb_sc.id=a.id)

優化方案3: 這種方案適用於delete語句太簡單而刪除資料較多的場合：

每次刪除一百條：

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

放在迴圈裡執行：

while(剩餘數量》0)

優化方案四：將要保留的資料存入一張臨時表，刪除原表再倒回來，這種操作最大的優勢在於降低表的水位線。

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