Skip to main content

TSQL Tuesday Misconceptions in SQL Server

I haven’t written a post for a TSQL Tuesday for a little while and I know that I’m late getting this one out too. This month’s party is being hosted by Sankar Reddy This topic  I think, maybe expect, really I know, that this topic has been covered already, I know that the Steve Jones has posted on this misconception already today and Paul Randal (Blog | Twitter) in his misconceptions series back in April covered this too much better than I have here so go and check out Paul’s post . The misconception I will look at here is - SQL Server will truncate the the transaction log after a full backup when the database is in full recovery mode. After my blog post around recovery models earlier today I think this maybe quite apt for me.
The misconception I am going to talk about here is SQL Server truncates the log of a database in full recovery mode when a full backup is taken. It doesn’t.
This one of the biggest problems I come across, a database in full recovery mode and regular full backups being taken but  no transaction log backups being taken. The log grows and grows and eventually fills up all drive space.
I tested this on a SQL Server 2000 instance earlier today and it holds true in that version too. SQL Server does not truncate the log when a full backup of a database is taken in full recovery mode, although I believe in versions gone by it may have been the case that a full backup truncated the log. If you know the exact version please leave a comment.

The Proof

I will create a database for the purpose of this test called demodb and then create a table in that database called t1 that will hold some data.

CREATE  TABLE t1 ( id int, amount int )

Firstly I will take a full database backup, the reason for backup is to ensures the database is in Full recovery mode and not in psuedo-simple mode.

BACKUP database demodb to disk = 'C:\BACKUP\demodb.bak'

If I then run a DBCC LOGINFO we can see that 1 of the 2 virtual log files (VLFs) are are active, they have a status of 2. I will cover VLF’s and the circular nature of the transaction log in a later posts, but for now DBCC LOGINFO shows some information on VLFs. a status of 2 means the VLF  is active, a status of 0 means that it is inactive or has been been truncated and can be used again.


I will then run some inserts on my table, all of which will be logged in the transaction log:

SET @i = 0

SET @j = 100000

WHILE @i < 10000

INSERT  INTO dbo.t1 ( id, Amount )
VALUES  ( @i, @j + @i )
SET @i = @i + 1

Running the DBCC LOGINFO command again,  you can see from the below diagram the log has grown and the number of active VLF’s has increased, I now have 103 VLF’s in my log and 102 are active, this means they cannot be re-used until they are marked as inactive and truncated.


I will then run another full backup of my database, to prove that a full backup of a database in full recovery mode does not truncate the log.

backup database demodb to disk = 'C:\BACKUP\demodb2.bak'

Running DBCC LOG info shows that the log has not been truncated by the full backup, the same number of VLFs are still active with a status of 2.

dbcc loginfo

The following results show that we still have 103 VLFs with 102 with a status of 2 – active.


If we then run a LOG backup and then run the DBCC LOGINFO command we can see that the log has been truncated and more of the VLF’s have been marked as inactive with a status of 0 and can now be reused.

backup log demodb to disk = 'C:\BACKUP\demodblog.trn'
dbcc loginfo


You will see that I still have 103 VLFs (more on that later) but now only one of those is active with a status of 2. The rest have been truncated and can be reused in the log. I hope that is helpful. I have rushed this post to get it out in time, I didn’t realise it was that time again until a couple of hours ago, if you find any inaccuracies in the post please let me know in the comments.


  1. Thanks for your posting, i am just a newbie in the internet business, need to learn a lot from the gurus
    windows 7 starter key


Post a Comment

Popular posts from this blog

SQL Server 2012 and Virtual Service Accounts

This post is written by David Postlethwaite
If you are using SQL Server 2012 you will probably have noticed that the default account for the SQL services has changed from that used in previous versions. With SQL 2005 and 2008 the default account for SQL service and SQL Agent service was “NT Authority\System”. This is one the built in accounts on a Windows machine, managed by the machine and selectable from a dedicated dropdown list

The Network Service account was introduced in Windows 2003 as an alternative to using the LocalSystem account, which has full local system privileges on the local machine, a major security concern.
The Network Service has limited local privileges easing these security concerns but when many services on a machine use the Network Service account it becomes harder to track which service is actually accessing resources and performing actions, because all the services are using the one Network Service account.
Also, this account, by default, has sysadmin per…

Always Encrypted

By David Postlethwaite

Always Encrypted is new features in SQL Server 2016 and it is also available in Azure SQL Database. Here you can encrypt columns in a table with a master key and a certificate so that they will appear as encrypted strings to those who don’t have the required certificate installed on their pc.
Once the certificate is installed on the computer then the unencrypted data can then be seen as normal.

The data passes from database to your application as the encrypted value, only the application with the correct certificate can unencrypt the data so it is secure across the wire. This will go some way to resolving the concern of people worried about putting their sensitive data on a shared server in the cloud such as Microsoft Azure and accessing the data across the Internet.

At the time of writing Always Encrypted is only supported with ADO.NET 4.6, JDBC 6.0 and ODBC 13.1 but expect other driver to become available.

The calling application (including SSMS) must also hav…

How to Setup Kerberos Correctly

David was in Copenhagen this weekend delivering his Kerberos talk Taming the Beast: Kerberos for the SQL DBA to SQL Saturday Denmark. I have had a quick chat with him via email since he got back and he said he had a great time. The event was very well attended with 280+ attendees and his talk was well attended.

I think David is planning submitting a few sessions to SQL Saturday events in Europe in the next few months so look out for him there and we'll keep you posted as to his whereabouts when schedules get finalised later in the year.

David has pre-recorded his Kerberos talk. You can watch on you tube and I have also embedded it in this post if you want to see what his kerberos talk  covers...

If we can help you with a SQL Sever problem visit our SQL Server Consulting page or contact us