Friday, May 30, 2008

FAQ : SQL Server DBA as a career option

This is a very common question in SQL Forums hence i thought to blog my view.

If you are a college passed out or a newbie in IT field, then I would suggest you to go for SQL Server MCTS (70-431) and MCITP DBA (70-443 and 70-444) certifications. Do not clear the certification for the sake of it. Learn the techniques and best practices mentioned in the syllabus. You must also go through the Virtual Courses available in Microsoft site. One great thing about Microsoft is , there are fantastic resource available in internet which comes free of cost. You just need to have MSN Live ID and login with that ID and refer the resource. Also must not forget download all the SQL OnDemand webcasts available in Microsoft site. Mind you, Webcast comes right from very experience folks who have been working for decades in this technology and also from the product teams who developed that products. Basically, reading , listening to experience people (webcast) and learn new technology will certainly give you a good launch in SQL Server. Always try to start with new product versions available (like at this point of time we have 2005 and 2008 CTP) in the market to be a leader rather than a follower.

For those who are already into IT field and wants to switch to DBA career can also more or less follow the same steps I have mentioned earlier. Key point in DBA for that matter any field, is Documentation. Try to document day to day activities and be process oriented. DBA is very risky at the same time secure job. Contradictory statements right?. Risky because you are working on production and data is nothing less than god for a DBA or you can say Data is your job. If anything goes wrong, it will may cost your job also. Secure, because the technology in Database generally not changes drastically. Consider one is working in VB 6 and one fine day he has been told to work in .Net 2.0. He will surely have tough time. But for a DBA to jump from SQL Server 2000 to 2005 or 2008 may be a smooth drive compared to other technologies.

SQL Server 2008 Virtual Lab
SQL Server 2005 Virtual Lab
SQL Server 2000 Virtual Lab

Microsoft ondemand Webcasts

Information about all the certification available SQL Server 2005

Wednesday, May 28, 2008

FAQ : What all are the recommended events for DTA / ITW workload

If you have provided same workload without different events and columns DTA may give you different report. For eg. if you have not included Duration column, DTA will tune the workload in the order they appear in the workload. IF the workload contains duration column , DTA will tune the events in the descending order of the Duration. So to get better result from DTA included recommended events and columns.

Events Recommended

FAQ : How to run DBCC DBREINDEX against all the user table in SQL Server

Simple, use undocumented stored procedure sp_msforeachtable. Since its undocumented there is no gurantee that it will be available in all the future versions. But in SQL Server 2000 and 2005 it works.

Use YourdatabaseName
EXEC sp_msforeachtable 'DBCC DBREINDEX( ''?'')'

FAQ : How triggers in SQL Server 2005 impact Tempdb

In SQL Server 2005 the triggers are implemented using Version store feature. In earlier versions (SQL Server 2000/ SQL 7), the trigger's Inserted and Deleted tables data were captured by reading Transaction log. But in SQL Server 2005, the Inserted and Deleted data is stored in Tempdb as row version. So, if you have more trigger in SQL Server 2005, it may impact Tempdb performance.

FAQ : How to truncate and shrink Transaction Log file in SQL Server

First of all truncation of transaction log is not a recommended practice. But it is unavoidable if you have not kept proper backup policy and recovery model for your database. Its always better to know the cause and prevention for the transaction log size issue. Refer the following KB for more info

Managing the Size of the Transaction Log File
Transaction Log Physical Architecture
Factors That Can Delay Log Truncation

Now coming to the point. If you have no space left with the drive where the Log file is kept and the size of the Transaction Log file is not manageable then its better to shrink the log.

Broadly , you have two steps here.
(a) Mark the inactive part of Trasaction log to release.
(b) Release the marked release portion of Transaction log to OS.

SQL Server 2005

-- Step 1 – Mark the inactive part of the log for release

Use YourDatabaseName
Backup Log YourDatabaseName With Truncate_Only

-- Step 2 - Release the marked space to OS

Declare @LogFileLogicalName sysname
select @LogFileLogicalName=Name from sys.database_files where Type=1
print @LogFileLogicalName

DBCC Shrinkfile(@LogFileLogicalName,100)

Note : If you have single log file the above mentioned script will work. IF you have multiple log file the change the script accordingly

SQl Server 2000

-- Step 1 – Mark the inactive part of the log for release

Use YourDatabaseName
Backup Log YourDatabaseName With Truncate_Only

-- Step 2 - Release the marked space to OS

Declare @LogFileLogicalName sysname
select @LogFileLogicalName=Name from sysfiles where filename like '%.ldf'
print @LogFileLogicalName

DBCC Shrinkfile(@LogFileLogicalName,100)

Note : If you have single log file and the extension of the log file is .LDF the above mentioned script will work. IF you have multiple log file the change the script accordingly

SQL Server 2008

In SQL Server this process have been changed. In 20008, just change the recovery model to simple and then use DBCC SHrinkfile command.

select name,recovery_model_desc from sys.databases
Alter database YourDatabaseName Recovery simple
Declare @LogFileLogicalName sysname
select @LogFileLogicalName=Name from sys.database_files where Type=1
print @LogFileLogicalName

DBCC Shrinkfile(@LogFileLogicalName,100)

Tuesday, May 27, 2008

FAQ : How to search for an object in all the databases

SQL Server 2005






SQL Server 2000






Note : Replace "YourSearchingObjectName" with the object name which you are searching in the select query

FAQ : Index Scan Vs Seek in SQL Server

There are five logical cum physical operators in SQL Server related to Index scan cum Seek and Table scan

(a) Table Scan
(b) Clustered Index Scan
(c) Clustered Index Seek
(d) Index Scan (Non- Clustered index Seek)
(e) Index Seek (Non- Clustered index Seek)

Table Scan
Table Scan retrieves all rows from the table (if you have no WHERE Conditions). Basically, before returning the rows, it traverse through all data pages related to the table. If you have where condition, though it travel through all the pages only those rows are returned which satisfy the conditions. When you do not have Clustered index on the table it does a table scan. In other words, both Clustered Index Scan (clustered index is nothing but the data itself) and Table Scan are same because in both method system traverse through all the data pages. Generally, you should avoid table scan.

Clustered Index Scan

Clustered Index Scan is nothing but horizontally traversing though the clustered index data pages. Clustered Index Scan return all the rows from the clustered index (Clustered index is nothing but Data). If you have where condition , only the satisfying rows are returned, but system traverse through all the data pages of the clustered index. Both Table scan and Clustered Index Scan are generally considered to be bad. But at times like if the table is small contains only few rows table or clustered index scan may be good also

Clustered Index Seek
Clustered Index Seek traverse vertically right down to the Data page where the requested data is stored. Basically any seek is vertically traversing though the B-Tree structure (as we all know the index is stored in B-tree structure in sql server). System does a Seek when it find a useful index and generally its done for highly selective query.

Index Scan or Non-Clustered Index Scan
As already said, Scan is horizontal traversing of B-Tree data pages. But in this case it horizontally traverse though the Non-Clustered index available. Its not same as Clustered index scan or Table Scan. In SQL Server , while reading execution plan you can find only Index Scan not Non-Clustered Index Scan. But you must read Index Scan as Non-Clustered Index Scan.

Index Seek or Non-Clustered Index Seek
As already mentioned Seek is Vertical traversing of B-Tree to the data page. But in Index seek it vertical traversing of Non-Clustered Index. Generally, its considered as the best option for high selective query.

FAQ : What is the difference between DELETE TABLE and TRUNCATE TABLE commands


• Less Transaction Log entry because TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction log and hence TRUNCATE is fast
• TRUNCATE apply Fewer table locks
• TRUNCATE is DDL Statement
• TRUNCATE Release the tables spaces to System
• TRUNCATE Can not have WHERE Conditions
• TRUNCATE does not fire trigger
• TRUNCATE reset the identity to 0 (if table have any)
• TRUNCATE Can not be used against the tables involved in TRUNCATE transactional replication or merge replication.
• TRUNCATE Can not be used against the table used in Indexed view
• TRUNCATE can not be used against the table that Are referenced by a FOREIGN KEY constraint.
• TRUNCATE commands are not tracked by DDL trigger

Note : TRUNCATE can be rollbacked. I have seen many place where its is mentioned that it can not


• DELETE FROM TABLE command logs each records in transaction log , hence DELETE is slow.
• DELETE apply more locks to the table
• DELETE is a DML command
• DELETE remove the records but will not release the space to the system
• DELETE can have WHERE conditions
• DELETE Can be used against table used transactional replication or merge replication
• DELETE Can be used in tables reference by a Foregin Key and tables involved in Indexed view

FAQ : How to move a physical file (MDF or LDF) from one location to another in SQL Server 2005

In SQL Server 2000, if you want to move a physical file of a database from one location to another , you have to detach and attach the database. In SQL Server 2005, this process has been made very simple, you need to take the database offline, alter the file path with the new one using Alter Database command and copy the database file to new location manually and finally take the database online. Simple

--Step 1 : Create Database
Create Database TestMoveFile and check the database file location
Select *From Sys.master_files where database_id=db_id('TestmoveFile')
--Step 2 : Alter Database and Set the db to offline
Alter Database TestMoveFile Set Offline
-- Step 3 : Move the physical file to new location
--Move the file to new location using dos command or Windows GUI

--Step 4 : Alter the database file path using Alter Database command
Alter Database TestMoveFile Modify File(Name='TestMoveFile',FileName='c:\TestmoveFile.mdf')

-- Step 5 : Set the database Online and check the file path
Alter database TestMoveFile Set Online
Select *From Sys.master_files where database_id=db_id('TestmoveFile')

Drop database TestMoveFile

FAQ : How can we know the progress of a maintenance command using DMVs

Its very common that, when we run DBCC maintenance command we would like to know how long will it take or how much percentage the process is completed. Here we go…

Select R.Session_id,R.Command,R.Percent_complete
sys.dm_exec_requests R
Inner Join
Sys.dm_exec_sessions S
on S.Session_id=R.Session_ID and S.IS_User_Process=1

This process is supported for DBCC CheckDB, DBCC CheckTable, DBCC CheckFilegroup,
DBCC IndexDefrag, DBCC Shrinkfile

Wednesday, May 21, 2008

FAQ : How to clear SQL Server cache / memory

Warning : Not to be used in production env.

In Development or tsesting env its very common that during performance tuning we do clear cache to get correct picture. It may also required that only a particular db related cache should be clear. here wo go...

(a) Clear entire procedure and databuffer cache

Checkpoint -- Write dirty pages to disk
DBCC FreeProcCache -- Clear entire proc cache
DBCC DropCleanBuffers -- Clear entire data cache

(b) Clear only a particular db procedure cache using undocumented DBCC command

Declare @DBID int
Select @DBID =db_id(‘YourDBname’)
DBCC FLUSHPROCINDB(@DBID) – Undocumented dbcc command to clear only a db proc cache

FAQ - How to use Stored Procedure in Select Query

Very common question in T-SQL Forums

The requirement is, need to use Stored procedure output (Exec SP) in Select query or in a Join. There is a method which may not be a recommended one. You can create a LoopBack Linked server to your own server and use OPENQUERY TO extract the data by EXECUTE Stored procedure.

LoopBack Linked Server

Linked servers can be defined to point back (loop back) to the server on which they are defined. Loopback servers are most useful when testing an application that uses distributed queries on a single server network.


('SQLOLEDB','Server=(local);TRUSTED_CONNECTION=YES;','set fmtonly off exec master.dbo.sp_who')


sp_addlinkedserver @server = N'MyLink',
@srvproduct = N' ',
@provider = N'SQLNCLI',
@datasrc = N'MyServer',
@catalog = N'AdventureWorks'

Note :
@server = N'MyLink' : It can not be your Server name. It is just a name and can be anything otherthan your actual servername. IF you give your server name in this parameter you will get an error as follows
Msg 15028, Level 16, State 1, Procedure sp_MSaddserver_internal, Line 89
The server 'LHI-115' already exists.

@datasrc = N'MyServer' : This parameter value has to be your server name or IP.

@catalog =N'AdventureWorks' : This is the database in which the Stored procedure exists.

OPENQUERY to Extract data from Loopback linkeserver

Select *from openquery([YourLoopbackServerName],'exec AdventureWorks.dbo.sptest')

Monday, May 05, 2008

How to export the output of a stored procedure or query to a text file using SQLCMD command in SQL Server 2005 and later versions

How to pump the output of a stored procedure or query to a text file using SQLCMD command in SQL Server 2005 and later versions

(a) Running SQLCMD command in Command prompt and create a file from the output
C:\> sqlcmd -Sservername -dmaster -Usa -Psa -q"se
lect *from sys.objects" -oC:\testoutput.txt

(b) Push the output of a query to a file using XP_CMDShell and SQLCMD command

Now what happens If we want to do this from Query Analyser. Simple , run the same command using XP_CMDShell.

(a) Open a Query Analyser in Management Studio
(b) Run the following query
Exec XP_CMDShell 'sqlcmd -Sservername -dmaster -Usa -Psa -q"select *from sys.objects" -oC:\testoutput.txt'

(c) Third and simple method
We all know the two methods mentioned above. But there is another method which is very simple and flexible. SQL Server Management Studio Query Editor has support for SQLCMD scripts authoring and execution. We can switch ON or Off this feature.

(a) Open Management Studio
(b) Tools Menu -->> Select Options  Query Execution  General  check the check box at the bottom of the page on the right side of the dialog “By default, open new queries in SQLCMD mode.”
(c) Press OK and save the settings
(d) Now , open a query analyzer and type the following
:out c:\queryresults.txt
:error c:\queryerror.txt
Select *from sys.objects
Select *from sys.objectsaaa

Note : In the above mentioned command the output of the first query will be pushed to C:\queryresults.txt and the error due to the second select statement will be pushed to c:\queryerror.txt

Disclaimer :

All the SQLCMD commands are not supported from Query Analyser. Following are the SQLCMD commands not supported in Query Analyser

(a):serverlist Lists local and SQL Servers on the network.

(b) :reset Discards the statement cache.

(c) :perftrace Redirects timing output to a file, stderr, or stdout. This is not supported because Query Editor doesn't have this concept.

(d) :listvar Lists the set SQLCMD scripting variables.

(e) :ed Edits the current or last executed statement cache.

(f) :help Shows the list of supported commands.

if you use any command in Query Analyser, Query Editor issues a non-fatal warning into the Messages tab during execution and continues on.

Summary :

All the three methods are equally useful. Many folks may not have observed the third method that is why i thought to blog this..