Drop/Delete/Truncate table

注意:这里说的delete是指不带where子句的delete语句
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

The difference in TRUNCATE and DELETE in Sql Server

I’ve answered this question many times, and answered it again this weekend.  What is the difference when doing a DELETE TableA instead of TRUNCATE TableA?  A common misconception is that they do the same thing.  Not so.  In fact, there are many differences between the two.

DELETE is a logged operation on a per row basis.  This means that the deletion of each row gets logged and physically deleted.

You can DELETE any row that will not violate a constraint, while leaving the foreign key or any other contraint in place.

TRUNCATE is also a logged operation, but in a different way.  TRUNCATE logs the deallocation of the data pages in which the data exists.  The deallocation of data pages means that your data rows still actually exist in the data pages, but the extents have been marked as empty for reuse.  This is what makes TRUNCATE a faster operation to perform over DELETE.

You cannot TRUNCATE a table that has any foreign key constraints.  You will have to remove the contraints, TRUNCATE the table, and reapply the contraints.

TRUNCATE will reset any identity columns to the default seed value.  This means if you have a table with an identity column and you have 264 rows with a seed value of 1, your last record will have the value 264 (assuming you started with value 1) in its identity columns.  After TRUNCATEing your table, when you insert a new record into the empty table, the identity column will have a value of 1.  DELETE will not do this.  In the same scenario, if you DELETEd your rows, when inserting a new row into the empty table, the identity column will have a value of 265.