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Home Database Mysql Tutorial ORACLE DELETE & TRUNCATE 區(qū)別

ORACLE DELETE & TRUNCATE 區(qū)別

Jun 07, 2016 pm 03:20 PM
amp delete oracle truncate the difference

用法: delete from tablename; truncate table tablename; 兩者 相同點: delete、truncate都能起到清除表數(shù)據(jù)功能; 兩者 不同點: delete語句用于表、視圖等;delete可以指定單條數(shù)據(jù)刪除;delete屬于DML操縱語言; truncate只能用于表;truncate刪除整表

用法:

delete from tablename;

truncate table tablename;

兩者相同點:

delete、truncate都能起到清除表數(shù)據(jù)功能;

兩者不同點:

delete語句用于表、視圖等;delete可以指定單條數(shù)據(jù)刪除;delete屬于DML操縱語言;

truncate只能用于表;truncate刪除整表數(shù)據(jù);truncate屬于DDL定義語言;

也就是說delete刪除之后數(shù)據(jù)可以在回滾,使用rollback;而truncate刪除之后數(shù)據(jù)時不可恢復的。

執(zhí)行效率:

truncate語句刪除效率遠高于delete語句。

兩者執(zhí)行原理:

delete:刪除一條數(shù)據(jù)之前首先copy這條數(shù)據(jù)到撤銷段,刪除幾百幾千萬條都先要copy到撤銷段,有人就問了,刪除之前為什么都要copy到撤銷段里面,別忘了,delete刪除之后還可以再使用rollback恢復的,恢復的數(shù)據(jù)就要根據(jù)日志文件到撤銷段里找到再回復。中間多一種copy操作,增加額外的一種安全保證,損失點效率是肯定的;

truncate語句直接進行刪除,無須copy到撤銷段,效率的提升是顯著的,但帶來的風險也是存在的,如誤刪除了數(shù)據(jù),該刪除數(shù)據(jù)是不可恢復的因為他是DDL,后臺隱式提交事務(wù)。用時需謹慎。

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