SQL SERVER - TRUNCATE and Delete

SQL SERVER - TRUNCATE Can’t be Rolled Back Using Log Files Aft... http://blog.sqlauthority.com/2007/12/26/sql-server-truncate-cant-be-rolle...

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SQL SERVER - TRUNCATE Can’t be Rolled Back Using Log Files After Transaction

Session Is Closed

December 26, 2007 by pinaldave

You might have listened and read either of following sentence many many times.

“DELETE can be rolled back and TRUNCATE can not be rolled back”.

OR

“DELETE can be rolled back as well as TRUNCATE can be rolled back”.

As soon as above sentence is completed, someone will object it saying either TRUNCATE can be or can not be rolled back. Let us make sure

that we understand this today, in simple words without talking about theory in depth.

While database is in full recovery mode, it can rollback any changes done by DELETE using Log files. TRUNCATE can not be rolled

back using log files in full recovery mode.

DELETE and TRUNCATE both can be rolled back when surrounded by TRANSACTION if the current session is not closed. If

TRUNCATE is written in Query Editor surrounded by TRANSACTION and if session is closed, it can not be rolled back but DELETE

can be rolled back.

Let us understand this concept in detail.

In case of DELETE, SQL Server removes all the rows from table and records them in Log file in case it is needed to rollback in future. Due

to that reason it is slow.

In case of TRUNCATE, SQL Server deallocates the data files in the table and records deallocation of the data files in the log files. If

deallocated data files are overwritten by other data it can be recovered using rollback. There is no guarantee of the rollback in case of

TRUNCATE. However, while using T-SQL following code demonstrates that TRUNCATE can be rolled back for that particular session.

First create test table which some data. Afterwards run following T-SQL code in Query Editor and test the effect of TRUNCATE on created

test table.

BEGIN TRAN

TRUNCATE TABLE TestTable

-- Following SELECT will return TestTable empty

SELECT * FROM TestTable

-- Following SELECT will return TestTable with original data

ROLLBACK

SELECT * FROM TestTable

Summary : DELETE can be recovered from log file always if full recovery mode is set for database. TRUNCATE may or may not be

recovered always from log files.

Reference : Pinal Dave (http://www.SQLAuthority.com)

Posted in Author Pinal, SQL, SQL Authority, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, T SQL, Technology | 11


2 of 8 2/18/2008 7:16 PM

Comments

11 Responses to “SQL SERVER - TRUNCATE Can’t be Rolled Back Using Log Files After Transaction Session

Is Closed”

on December 27, 2007 at 2:51 pm1 kris

i have table with the data like below:

ids_cloumn

========================

1 , 2 , 4 , 23 , 12 , 11 , 9 , 8 , 7 , 10 ,

1 , 3 , 4 , 21 , 16 , 12 , 19 , 10 ,

2 , 3 , 6 , 24 , 15 , 18 , 9 , 8 ,

3 , 5 , 8 , 27 , 14 , 19 , 39 , 7 , 13 ,

1 , 7 , 9 , 28 , 19 , 11 , 29 , 8 ,

i am having a list of nos with me (coming from another query)

ex1: 1, 3 & 23

ex2 : 1, 6, 34 & 23

now i want to retrieve the list of rows which is having any of the nos.

is there any other way with out using “LIKE %”

If i am using like i need to write so many OR conditions based on the inputs.

thanks

1.

on December 29, 2007 at 7:39 am2 Andrew Chen

The ping back doesn’t seem to work. Anyway I don’t think a commited delete statement can be rolled back either. The deleted data

can be restored from backup only.

2.

on December 29, 2007 at 9:10 am3 pinaldave

Hi Andrew,

Thanks for comment. I have tested it earlier and delete can be rolled back using log files even though it is committed. You will have to

use point of time restore using log files. However, Truncate can not be restored using log files. I tried to convey this message in title of

this post.

I appreciate your regular participation in this blog. Your comments are always valuable.

Regards,

Pinal Dave ( http://www.SQLAuthority.com )

3.

on December 29, 2007 at 5:50 pm4 Andrew Chen

Hi Pinal,

Thanks for replying. I feel confuse when you mention roll back using log file together with the “Roll Back” statement. Roll back

statement only rolls back the current open transaction in current database session. It can not roll back a committed transaction.

Roll back using log file however will roll back many transactions. we can not roll back a specific delete that way. Everthing done after

the delete will also get rolled back. I got the impression from your post that we can roll back a specific delete statement using log file.

4.


3 of 8 2/18/2008 7:16 PM

As you point out roll back using log file is to restore a database to a point of time

on December 29, 2007 at 6:27 pm5 Andrew Chen

I forgot to mention that I think point in time restore using log file can also roll back “TRUNCATE”.

5.

on December 29, 2007 at 6:37 pm6 pinaldave

Andrew,

Point in time restore using log file can not roll back TRUNCATE if the data file which is deallocate is over-written. That is the

difference.

That is why I used last statement in post “TRUNCATE may or may not be recovered always from log files.”

Kind Regards,

Pinal Dave ( http://www.SQLAuthority.com)

6.

on December 30, 2007 at 3:27 am7 Andrew Chen

Pinal,

Sorry for being stubborn. But as far as I know in a point in time restore. A full database backup has to be be restored first and then the

log backup is actually used to “Roll Forward” the database to the point in time when the data deleted or turncated were still exist. So it

would not be a problem whether or not the data file got over written because it will be re-created in database restore anyway.

Maybe I miss unstand something but if you can share an example of using log file to “Roll Back” a commited DELETE then it can

make things clear. I believe it would be something a lot of DBAs like know about

Appreciated

Andrew

7.

on January 1, 2008 at 11:36 pm8 Alexandr Volok

I’m completely agree with Andrew. Truncate table not just safe for rollback in transaction scope. It still possible undo truncation of

table using restore log and stopat.

There just reproduction code:

– STEP 1. Creating DB

SET NOCOUNT ON

CREATE DATABASE TEST

GO

ALTER DATABASE TEST SET RECOVERY FULL

USE TEST

GO

— STEP 2. Creating table with some data (100 rows)

CREATE TABLE data (f1 INT IDENTITY)

GO

INSERT data DEFAULT VALUES

GO 100

– STEP 3. Performing full backup

BACKUP DATABASE TEST TO DISK=’c:\test_full.bak’ WITH INIT

GO

8.


4 of 8 2/18/2008 7:16 PM

– STEP 4. Performing log backup

DECLARE @date DATETIME

– variant1: Date mark just before truncation

SET @date = DATEADD(s, 0, GETDATE())

– variant2: Date mark after truncation

– SET @date = DATEADD(s, 1, GETDATE())

TRUNCATE TABLE data

WAITFOR DELAY ‘00:00:03′

BACKUP LOG TEST TO DISK=’c:\test_log.bak’ WITH INIT

– STEP 5. Restoring Test DB.

USE MASTER

RESTORE DATABASE TEST FROM DISK=’c:\test_full.bak’ WITH NORECOVERY, REPLACE

RESTORE LOG TEST FROM DISK=’c:\test_log.bak’ WITH RECOVERY, STOPAT=@date

GO

USE TEST

GO

select COUNT(*) AS CNT FROM data

go

USE MASTER

DROP DATABASE TEST

p.s. Please, delete my previous comment :))

on January 2, 2008 at 12:26 pm9 pinaldave

Hi Alexandr Volok and Andrew,

Both of yours comment makes this article very valuable. Please continue writing your opinion and comments.

Kind Regards,


9.

on January 6, 2008 at 8:12 am10 Andrew Chen

Acutally there are third party transaction Log reading Tools that are able to read transaction logs and allow you undone commited

statements or transactions. Tuncate satement may or may not be undone using such tools. SQL server itselft does not provide this

capability.

Siusic Dot Com

10.

on January 6, 2008 at 10:29 am11 pinaldave

Hello Andrew,

Thank you for posting clarification and as well we are in agreement. I respect your additional research as well writing about this on

your blog http://www.siusic.com/wphchen/recover-data-using-transaction-log-144.html

You are always welcome to comment and participate here to help community.

Kind Regards,


11.

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About Pinal Dave

Pinalkumar Dave is the author of over 400 SQL Server articles. He has five years experience as Principal Database Administrator in

MS SQL Server 2005/2000 and ColdFusion MX. He has a Masters of Science degree in Computer Networks, along with MCDBA,

MCAD(.NET) and ColdFusion Advanced MX Certifications.

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