Delete、Drop、Truncate的比较

相同点:truncate和不带where子句的delete, 以及drop都会删除表内的数据 

不同点: 
1. truncate和 delete只删除数据不删除表的结构(定义) 
drop语句将删除表的结构被依赖的约束(constrain),触发器(trigger),索引(index); 依赖于该表的存储过程/函数将保留,但是变为invalid状态. 

2.delete语句是DML,这个操作会放到rollback segement中,事务提交之后才生效;如果有相应的trigger,执行的时候将被触发. 
truncate,drop是DDL, 操作立即生效,原数据不放到rollback segment中,不能回滚. 操作不触发trigger. 

3.delete语句不影响表所占用的extent, 高水线(high watermark)保持原位置不动 
显然drop语句将表所占用的空间全部释放 
truncate 语句缺省情况下见空间释放到 minextents个 extent,除非使用reuse storage; truncate会将高水线复位(回到最开始). 

4.速度,一般来说: drop> truncate > delete 

5.安全性:小心使用drop 和truncate,尤其没有备份的时候.否则哭都来不及 
使用上,想删除部分数据行用delete,注意带上where子句. 回滚段要足够大. 
想删除表,当然用drop 
想保留表而将所有数据删除. 如果和事务无关,用truncate即可. 如果和事务有关,或者想触发trigger,还是用delete. 
如果是整理表内部的碎片,可以用truncate跟上reuse stroage,再重新导入/插入数据 

附加DDL,DML,DCL区别： 
DDL is Data Definition Language statements. Some examples: 

* CREATE - to create objects in the database 
* ALTER - alters the structure of the database 
* DROP - delete objects from the database 
* TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed 
* COMMENT - add comments to the data dictionary 
* GRANT - gives user's access privileges to database 
* REVOKE - withdraw access privileges given with the GRANT command 

DML is Data Manipulation Language statements. Some examples: 

* SELECT - retrieve data from the a database 
* INSERT - insert data into a table 
* UPDATE - updates existing data within a table 
* DELETE - deletes all records from a table, the space for the records remain 
* CALL - call a PL/SQL or Java subprogram 
* EXPLAIN PLAN - explain access path to data 
* LOCK TABLE - control concurrency 

DCL is Data Control Language statements. Some examples: 

* COMMIT - save work done 
* SAVEPOINT - identify a point in a transaction to which you can later roll back 
* ROLLBACK - restore database to original since the last COMMIT 
* SET TRANSACTION - Change transaction options like what rollback segment to use