Difference between TRUNCATE, DELETE and DROP commands

delete

The DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire.

truncate

TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUCATE is faster and doesn't use as much undo space as a DELETE.

drop

The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.

1>TRUNCATE is a DDL command whereas DELETE is a DML command.

2>TRUNCATE is much faster than DELETE.

Reason:When you type DELETE.all the data get copied into the Rollback Tablespace first.then delete operation get performed.Thatswhy when you type ROLLBACK after deleting a table ,you can get back the data(The system get it for you from the Rollback Tablespace).All this process take time.But when you type TRUNCATE,it removes data directly without copying it into the Rollback Tablespace.Thatswhy TRUNCATE is faster.Once you Truncate you cann't get back the data.

3>You cann't rollback in TRUNCATE but in DELETE you can rollback.TRUNCATE removes the record permanently.

4>In case of TRUNCATE ,Trigger doesn't get fired.But in DML commands like DELETE .Trigger get fired.

5>You cann't use conditions(WHERE clause) in TRUNCATE.But in DELETE you can write conditions using WHERE clause

6> TRUNCATE command resets the High Water Mark for the table but DELETE does not. So after TRUNCATE the operations on table are much faster.

   If you're removing a large quantity of data (e.g. debug log table) then it makes sense to use truncate to clear the table if you're not worried about needing to retrieve it as part of a transaction - remember that you are stretching your undo tablespace to cover the whole of the data you are looking to delete if you using the DELETE FROM... command, only to immediately wipe that out again when you commit - for what purpose? A lot more processing effort when all you want to do is clear a table of its contents.

   On the minus side, if you have a foreign key constraint referring to the table you are trying to truncate, this won't work - even if the referring table has no data in it! This is because the foreign key checking is done with DDL rather than DML. This can be got around by temporarily disabling the foreign key constraint(s) to the table.

   With dropping a table, also bear in mind that you lose any associated grants / constraints etc. and if you want to recreate the table, you would need to recreate these - and it may be that the user dropping the table may not have the rights to re-grant etc. - so dropping is always an extreme measure! If you're constantly dropping / recreating you should probably be using temporary tables for one-off jobs or global temporary tables for frequently running processes.

评论

1 楼 mikixiyou 2012-06-11  

自己写的吗？挺不错的。