How do you clear the SQL Server transaction log?

Question

I'm not a SQL expert, and I'm reminded of the fact every time I need to do something beyond the basics. I have a test database that is not large in size, but the transaction log definitely is. How do I clear out the transaction log?

Answer 1

DISCLAIMER: Please read comments below carefully, and I assume you've already read the accepted answer. As I said nearly 5 years ago:

if anyone has any comments to add for situations when this is NOT an adequate or optimal solution then please comment below

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• Right click on the database name.

• Select Tasks → Shrink → Database

• Then click OK!

I usually open the Windows Explorer directory containing the database files, so I can immediately see the effect.

I was actually quite surprised this worked! Normally I've used DBCC before, but I just tried that and it didn't shrink anything, so I tried the GUI (2005) and it worked great - freeing up 17 GB in 10 seconds

In Full recovery mode this might not work, so you have to either back up the log first, or change to Simple recovery, then shrink the file. [thanks @onupdatecascade for this]

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PS: I appreciate what some have commented regarding the dangers of this, but in my environment I didn't have any issues doing this myself especially since I always do a full backup first. So please take into consideration what your environment is, and how this affects your backup strategy and job security before continuing. All I was doing was pointing people to a feature provided by Microsoft!

Answer 2

To Truncate the log file:

• Backup the database
• Detach the database, either by using Enterprise Manager or by executing : Sp_DetachDB [DBName]
• Delete the transaction log file. (or rename the file, just in case)
• Re-attach the database again using: Sp_AttachDB [DBName]
• When the database is attached, a new transaction log file is created.

To Shrink the log file:

• Backup log [DBName] with No_Log

• Shrink the database by either:

  Using Enterprise manager :- Right click on the database, All tasks, Shrink database, Files, Select log file, OK.

  Using T-SQL :- Dbcc Shrinkfile ([Log_Logical_Name])

You can find the logical name of the log file by running sp_helpdb or by looking in the properties of the database in Enterprise Manager.

Answer 3

DB Transaction Log Shrink to min size:

1. Backup: Transaction log
2. Shrink files: Transaction log
3. Backup: Transaction log
4. Shrink files: Transaction log

I made tests on several number of DBs: this sequence works.

It usually shrinks to 2MB.

OR by a script:

DECLARE @DB_Name nvarchar(255);
DECLARE @DB_LogFileName nvarchar(255);
SET @DB_Name = '<Database Name>';               --Input Variable
SET @DB_LogFileName = '<LogFileEntryName>';         --Input Variable
EXEC 
(
'USE ['+@DB_Name+']; '+
'BACKUP LOG ['+@DB_Name+'] WITH TRUNCATE_ONLY ' +
'DBCC SHRINKFILE( '''+@DB_LogFileName+''', 2) ' +
'BACKUP LOG ['+@DB_Name+'] WITH TRUNCATE_ONLY ' +
'DBCC SHRINKFILE( '''+@DB_LogFileName+''', 2)'
)
GO

Answer 4

The SQL Server transaction log needs to be properly maintained in order to prevent its unwanted growth. This means running transaction log backups often enough. By not doing that, you risk the transaction log to become full and start to grow.

Besides the answers for this question I recommend reading and understanding the transaction log common myths. These readings may help understanding the transaction log and deciding what techniques to use to "clear" it:

From 10 most important SQL Server transaction log myths:

Myth: My SQL Server is too busy. I don’t want to make SQL Server transaction log backups

One of the biggest performance intensive operations in SQL Server is an auto-grow event of the online transaction log file. By not making transaction log backups often enough, the online transaction log will become full and will have to grow. The default growth size is 10%. The busier the database is, the quicker the online transaction log will grow if transaction log backups are not created Creating a SQL Server transaction log backup doesn’t block the online transaction log, but an auto-growth event does. It can block all activity in the online transaction log

From Transaction log myths:

Myth: Regular log shrinking is a good maintenance practice

FALSE. Log growth is very expensive because the new chunk must be zeroed-out. All write activity stops on that database until zeroing is finished, and if your disk write is slow or autogrowth size is big, that pause can be huge and users will notice. That’s one reason why you want to avoid growth. If you shrink the log, it will grow again and you are just wasting disk operation on needless shrink-and-grow-again game

Answer 5

Database → right click Properties → file → add another log file with a different name and set the path the same as the old log file with a different file name.

The database automatically picks up the newly created log file.

Answer 6

Below is a script to shrink the transaction log, but I’d definitely recommend backing up the transaction log before shrinking it.

If you just shrink the file you are going to lose a ton of data that may come as a life saver in case of disaster. The transaction log contains a lot of useful data that can be read using a third-party transaction log reader (it can be read manually but with extreme effort though).

The transaction log is also a must when it comes to point in time recovery, so don’t just throw it away, but make sure you back it up beforehand.

Here are several posts where people used data stored in the transaction log to accomplish recovery:

• How to view transaction logs in SQL Server 2008

• Read the log file (*.LDF) in SQL Server 2008

 

USE DATABASE_NAME;
GO

ALTER DATABASE DATABASE_NAME
SET RECOVERY SIMPLE;
GO
--First parameter is log file name and second is size in MB
DBCC SHRINKFILE (DATABASE_NAME_Log, 1);

ALTER DATABASE DATABASE_NAME
SET RECOVERY FULL;
GO

You may get an error that looks like this when the executing commands above

“Cannot shrink log file (log file name) because the logical log file located at the end of the file is in use“

This means that TLOG is in use. In this case try executing this several times in a row or find a way to reduce database activities.