A way to Shrink Data File in SQL Server

Problem Statement

We recently wanted to drop few obsolete tables those are really big ones in my SQL Server databases. Post clean up, meaning dropping the tables, we would like to reclaim the space it occupied to OS and reduce the cost associated with. What are the best options in such situations?

DBA’s Ponderings

Usually, DBAs would never support shrinking of data file due to several reason. They consider about their system performance than the disk cost, especially, now days, the worry on cost associated must be an outdated belief. Shrink can cause many bad on your databases and some of them are listed as below:

1. It increases the fragmentation.

2. It will have a system performance issues due to heavy fragmentation.

3. While running shrink a data file, it may have dangerous locking issues. This must be an offline activity.

4. If shrink file on a LOB data, that would slow down the process badly.

Due to all the above reasons, we should always keep a long distance to shrink file operations. Thats not our best friend for our system.

Then, How do we shrink data file?

Then, What we do if its mandatory to do shrink the data file as our initial Problem Statement?

If its mandatory or meaningful or sensible, then you can do with few extreme considerations. In one of my case, I had to clean up almost 1000 tables from a database and I had to do this for almost 2000 databases. It had almost 4GB data to be cleared from a single database resulting a whooping amount of cost save! This is a real example where shrink file is important or a valid reason to do so. Myself & one of my colleague “Pascal Benguigui” have come up with an approach here as below.

Let us look at the options/considerations to apply shrink file as below. It may vary for others, but its more of our deliberate thoughts.

1. Understand the size of data that you wanted to eliminate. To understand, I would suggest you to take backup and restore your database to local and remove the data followed by shrink file and then identify the current size of data file.

2. If the file size if more than 1000 MB, we can consider shrinkfile, otherwise, it may not be so efficient.

3. If the file size is more than 1000 MB, let us also consider the percentage of free space. If free space percentage is lesser than 10%, it may not be so efficient again.

4. Let us also shrink the file up to the used space and 10 percentage added to the space. This must be a good number, but depends!, it may or may not suitable for all cases. A detailed analysis can be carried out and define as per your environment leaving it to the respective DBA’s descretion.

Shrink Script

CREATE OR ALTER procedure Shrink_DataFile @db varchar(200)

declare @f_name varchar(200)
declare @f_file varchar(500)
declare @f_size int
declare @f_used int
declare @pct_free int
declare @f_newsize int
declare @sql_query nvarchar(2000)
declare @sql_param nvarchar(200)

print 'Datafile SHRINK requested on database ['+@db+'] : '
declare cur_name cursor for 
select name from sys.sysaltfiles where dbid=db_id(@db) and groupid=1 and CONVERT(sysname,DatabasePropertyEx(@db,'Updateability'))='READ_WRITE' order by fileid asc

open cur_name
fetch next from cur_name into @f_name

while @@fetch_status=0

       select @sql_param =N'@f_size_out int OUTPUT,@f_used_out int OUTPUT,@f_file_out varchar(500) OUTPUT'
       select @sql_query=N'use ['+@db+']; select @f_size_out=CAST(s.size/128.0 AS int) , @f_used_out=CAST(FILEPROPERTY(, ''SpaceUsed'')/128.0 AS int),@f_file_out=filename from sys.sysfiles s where groupid=1 and name='''+@f_name+''''
       EXEC sp_executesql @sql_query, @sql_param, @f_size_out=@f_size OUTPUT, @f_used_out=@f_used OUTPUT,@f_file_out=@f_file OUTPUT ;
       select @pct_free=(@f_size-@f_used)*100/@f_size

       if @f_size >1000 and @pct_free>10
             -- shrink should keep 10% free space and be a multiple of 100 higher
             select @f_newsize=@f_used*1.1
             select @f_newsize=ceiling(cast(@f_newsize as decimal(38,2))/100)*100
             if @f_newsize < @f_size
                    print 'File "'+@f_file+' (size '+convert(varchar,@f_size)+' MB) " will be shrinked to '+convert(varchar,@f_newsize)+' MB ...'
                    select @sql_query=N'USE ['+@db+']; 
                    DBCC SHRINKFILE (N'''+@f_name+''' , '+convert(varchar,@f_newsize)+')  WITH NO_INFOMSGS'
                    print @sql_query
                    exec sp_executesql @sql_query

                    select @sql_param =N'@f_size_out int OUTPUT'
                    select @sql_query=N'use ['+@db+']; select @f_size_out=CAST(s.size/128.0 AS int) from sys.sysfiles s where groupid=1 and name='''+@f_name+''''
                    EXEC sp_executesql @sql_query, @sql_param, @f_size_out=@f_size OUTPUT;
                    print '... New size for file "'+@f_file+'" shrinked is '+convert(varchar,@f_size)+' MB
                    print 'NO SHRINK : New size estimated for file "'+@f_file+'" ('+convert(varchar,@f_newsize)+' MB) not lower than the current file size ('+convert(varchar,@f_size)+' MB)'

             print 'NO SHRINK : Size requirements not valid to shrink file "'+@f_file+' (size '+convert(varchar,@f_size)+' MB / free space '+convert(varchar,@pct_free)+' %)

       fetch next from cur_name into @f_name
       close cur_name
deallocate cur_name

if @f_file is null
       print 'database ['+@db+'] not existing or not in READ-WRITE mode


Post shrinkfile, we would expect lots of fragmentations. This can severely impact the system performance. We need to make sure that we need to avoid fragmentation as quickly as possible by running INDEX rebuild/reorganize. There is a well defined index maintenance solution defined by Ola Hallengren that can be used for the same.

If you have to plan it for many number of databases, you need to consider this to happen in multiple batches to ensure that this process does not hurt the system. There may be a chance that your immediate backup jobs taking longer time than usual. Importantly, we need to make sure the shrink followed by index defragmentation is planned at application off peak hours, otherwise it might end up with larger number of blocking/locking scenarios.

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How to get FQDN (fully qualified domain name) from SQL Server?

A fully qualified domain name (FQDN) is referred to as an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS).

You can see this information easily throughput Control Panel – > System and Security – > System – > Look out “Computer name, domain and workgroup settings”

But, I was looking for some way to identify this information without switching my windows (actually I was working on something when I was asked this information) .

EXEC master..xp_loginconfig 'Default Domain'

DECLARE @rootkey varchar(100) = 'HKEY_LOCAL_MACHINE',
@key varchar(100) = 'SYSTEM\ControlSet001\Services\Tcpip\Parameters\',
@value_name varchar(100) = 'Domain',@Domain_Name sysname -- output parameters
--Using xp_regread
EXEC master..xp_regread @rootkey = @rootkey, @key = @key, @value_name=@value_name
--Using xp_instance_regread
EXECUTE master.sys.xp_instance_regread
    @rootkey = @rootkey, @key = @key, @value_name=@value_name

/* Incase you need to assign to a variable*/
EXEC master..xp_regread @rootkey = @rootkey, @key = @key, @value_name=@value_name,@value=@Domain_Name OUTPUT 
SELECT @Domain_Name 'Domain Name'

Set @Domain_Name = ''

EXECUTE master.sys.xp_instance_regread
    @rootkey = @rootkey, @key = @key, @value_name=@value_name,@value=@Domain_Name OUTPUT 

SELECT @Domain_Name 'Domain Name'
Since, its domain information, I am not providing the screenshot, you may try it out yourself and share your thoughts.

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How to get Disk space information in SQL Server

Problem Statement:

While troubleshooting an issue in SQL Server, I wanted to know the space information of the server. Since we did not have access to the server, we have come up with a simple script as below. I am sharing the script here hoping this helps you in similar situation.


declare @svrName varchar(255)
declare @sql varchar(400)
--User Options(By default Server name)
set @svrName = Cast(SERVERPROPERTY('MachineName') as varchar(255))
set @sql = 'powershell.exe -c "Get-WmiObject -ComputerName ' + QUOTENAME(@svrName,'''') 
	+ ' -Class Win32_Volume -Filter ''DriveType = 3'' '+
	'| select name,capacity,freespace |foreach{$''|''+$_.capacity/1048576+''%''+$_.freespace/1048576+''*''}"'

--creating a temporary table
(line varchar(255))
--inserting disk name, total space and free space value in to temporary table
insert #Details
EXEC xp_cmdshell @sql

--Retrieve the Capacity values in GB from PS Script Details
select rtrim(ltrim(SUBSTRING(line,1,CHARINDEX('|',line) -1))) as drivename
      (CHARINDEX('%',line) -1)-CHARINDEX('|',line)) )) as Float)/1024,0) as 'capacity(GB)'
      (CHARINDEX('*',line) -1)-CHARINDEX('%',line)) )) as Float) /1024 ,0)as 'freespace(GB)'
from #Details
where line like '[A-Z][:]%'
order by drivename

--Drop the temporary table
drop table #Details

See Also:

Using windows PowerShell to get the server disk space

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Fun with SQL – Find out numbers where adjacent digit differs by 1

Today, I have come through a post by one of my good friends and one of the most famous SQL Server experts in India – Madhivanan.

The question was : “Given a number N, write a code to print all positive numbers less than N in which all adjacent digits differ by 1” Here is my attempt to solve this using SQL Server T-SQL
--Provide input value 
declare @number int
set @number=105
--Create a temporary table to store numbers
Drop table if exists #temp
create table #temp (number int)

;with ctebuilder as(
    row_number () over (order by (select null)) as sno 
    sys.sysobjects as t1 cross join sys.sysobjects as t2 ) 
Insert into #temp(number) Select sno From ctebuilder where sno<=@number

;with cte as(
    t1.number number, 
    cast(substring(cast(t1.number as varchar(20)),t2.number,1) as int) as number2, 
	row_number () over(partition by t1.number order by (select null) asc) Rn
from #temp as t1 cross join #temp as t2
where t2.number <= len(t1.number) and t1.number <=@number
Select number From(
 Select * ,LAG(number2,1) OVER (partition by number
		ORDER BY (select NULL) asc
	) previous_val From cte
	) A group by number 
having count(case when previous_Val - number2 in (1,-1) then 1 Else NULL End )  
= count(number2)-1

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Database Properties – Difference between GUI and sys.master_files in SQL Server

Few days back, One of my colleagues was asking why there is a discrepancy in GUI and system table for Database Properties -> File wizard. Here is an example similar to what he showed me.

GUI Screenshot

SSMS Query Screenshot

My first response was GUI is a user friendly and Query results is a SQL Server Professional friendly. There will be lots of differences like this between GUI and actual table how it saved.But, those are neither discrepancies nor defects, that is how it is by design.

To know this difference, sys.master_files is a system catalog view that represents properties of each file of your database ( data and log). size is always representing in 8 KB pages. Refer the link for more details for other properties.

So, to make him understand, I changed his query a bit to get both looks equal as below.

Select [Logical Name], A.type_desc [File Type], ISNULL(B.Name, 'Not Applicable') 'Filegroup',
(size*8)/1024 [Size (MB)],
'By ' + Cast((growth*8)/1024 as varchar(max)) + 'MB, ' + 
Case when max_size = -1 then 'Unlimited' Else Cast(Max_size as varchar(max)) End + ' MB' [AutoGrowth/MaxSize],
substring(physical_name,0, Len(Physical_name)-charindex('\',reverse(Physical_name),0)+1) [Path],
Right(physical_name,charindex('\',reverse(Physical_name),0)-1) [File Name]
From sys.master_files A
Left Join sys.filegroups B on A.data_space_id = B.data_space_id
where db_name(database_id) = 'DBATools'

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