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Friday, July 22, 2011

What does resourcedb contain ?

Many a times this question has been asked (either in interviews or just out of curiosity) that what does resourcedb contain and why is it so important to SQL Server ?

Normally we can't see it and hence can't use it .However, there is a way you can use resource database .But be careful. If you mess up anything you might end up paying a heavy cost.

Since we are discussing about this ,there is one more point that I would like to touch here .Starting from SQL 2005 there are no system tables but DMVs for us to query .However, if you query sys.objects and filter it on type ='S' ,you will notice a lot of system tables listed in the output .So there are system tables and we can see them .But if you try to query them ,you will get an annoying 208 error stating that the object does not exist which is not correct.So in this post we will see how to query resource database and also in a similar manner other system and user database .

Let us see how can we use resource database and also query system tables.Start SQL Server with -m switch ( in single user mode ) . There are 2 ways :

1) Through services console (after adding -m do not click on OK but click on start )

2) Through DOS prompt

Once SQL Server has been started in single user mode , we can make only one connection . We will connect to SQL Server using DAC .DAC option can only be used in sqlcmd utility and not in OSQL or ISQL .Again there are two ways to do this .But before trying to attempt for DAC connection make sure you have enabled remote admin connection option via sp_configure (you can see the run value of 1):

1) connecting to SQL Server with DAC (using SQLCMD) using -A option

2)Connecting via MGT Studio
Open Management studio , It will prompt you to enter the instance name . Just before the instance name add admin:

you might or might not get this error :

If you get this error then click OK ( this error window will go away ) and then instead of Connect click on cancel .
you will see a clean screen like below :

Click on new query

You will again see the same connection popup :

This time click on connect and it will work :) .A new query window will open even though you will not see the databases in the left hand side pane but the connection is there and working ...

Run the query 'use mssqlsystemresource' and press F5 :) ...It will work :

Also if you query sys.sysdbreg sytem table (an alternative for sys.sysdatabases DMV )you will see the resource database :

This database is currently in read-only mode (trust me :-) ) . If you want to cross check this run dbcc shrinkdatabase (mssqlsystemresource) and you will get to know .
You can set it to read_write mode though by running : alter database mssqlsystemresource set read_write.Now the very fact that this table is Read-Only and that we cannot take the backup of this database ,proves that this database might not contain very important information ...if you query the tables you will see that certain static information is stored which the engine might be using from time to time .Something like we store the values in a temp table or a variable .So coming back to the original question , resource database contain a lot of static information which the engine might need from time to time for its internal use .

At the end you might ask "Whats the need to touch the system tables in the database ?"
The answer is : We normally do not need to do this (especially Resource DB ) but there are other databases (system as well as user ) having some information we can use to resolve some issues (by updating those tables as needed ). And to resolve those issues , we need to login in this way ....

Hope you have found it interesting .But remember , BE VERY CAREFUL WHEN YOU TRY TO PLAY AROUND WITH SYSTEM TABLES (as i said in the beginning).
Happy Learning !!


Anonymous said...

What about supportability after changing something in system tables???

Abhay said...

I am not very much sure about this But I am sure that MS will atleast try to assist on the best effort basis (since I worked with MS in the past and have been through similar such situations)and if we document it when we make those changes to show it to the MS Support , that will be even better as then something can be reverted back to the supportable level again ..The only thing is that MS will not gurantee solutions and will not take the blame if it cannot resolve such issues.

But they are the best , mostly end up going an extra mile to resolves such issues (atleast I saw almost all of them getting resolved when I was in MS for more than 2 yrs)