Sql-server – How to reclaim the unused Space in a database

When using the DBCC SHRINKDATABASE statement, you cannot shrink a whole database to be smaller than its original size. Therefore, if a database was created with a size of 10 MB and grew to 100 MB, the smallest the database could be reduced to is 10 MB, even if all the data in the database has been deleted.

However, you can shrink the individual database files to a smaller size than their initial size by using the DBCC SHRINKFILE statement. You must shrink each file individually, instead of trying to shrink the whole database.

This can be accomplished somewhat easily by using a "loop back" Linked Server that has the 'remote proc transaction promotion' property set to false, which avoids the Cannot perform a shrinkfile operation inside a user transaction error by side-stepping the implicit transaction started by the INSERT...EXEC operation. The Linked Server will otherwise only require that the 'rpc out' property be set to true. The 'data access' and 'rpc' properties do not need to be enabled (at least not for me).

Create the Linked Server

USE [master];

DECLARE @LinkedServerName sysname;
SET @LinkedServerName = N'TestDBCC';

IF (NOT EXISTS (SELECT srv.name
                FROM   sys.servers srv
                WHERE  srv.server_id <> 0
                AND    srv.name = @LinkedServerName))
BEGIN
  EXEC [master].dbo.sp_addlinkedserver
    @server = @LinkedServerName, @srvproduct=N'', @provider=N'SQLNCLI11', @datasrc=N'DALI';
  EXEC [master].dbo.sp_addlinkedsrvlogin
    @rmtsrvname=@LinkedServerName,@useself=N'True',@locallogin=NULL,
    @rmtuser=NULL,@rmtpassword=NULL;
END;

--EXEC [master].dbo.sp_serveroption @server=@LinkedServerName,
--  @optname=N'data access', @optvalue=N'true';
--EXEC [master].dbo.sp_serveroption @server=@LinkedServerName,
--  @optname=N'rpc', @optvalue=N'true'; -- is_remote_login_enabled
EXEC [master].dbo.sp_serveroption @server=@LinkedServerName,
  @optname=N'rpc out', @optvalue=N'true';
EXEC [master].dbo.sp_serveroption @server=@LinkedServerName,
  @optname=N'remote proc transaction promotion', @optvalue=N'false';
GO

Use the Linked Server via EXEC() AT

CREATE TABLE #ShrinkFileResults
(
    [DBID] SMALLINT,
    [FileID] INT,
    [CurrentSize] INT,
    [MinimumSize] INT,
    [UsedPages] INT,
    [EstimatedPages] INT
);

INSERT INTO #ShrinkFileResults
  ([DBID], [FileID], [CurrentSize], [MinimumSize], [UsedPages], [EstimatedPages])
  EXEC(N'
     USE [tempdb];
     DBCC SHRINKFILE(N''tempdev'', 0, TRUNCATEONLY);
  ') AT [TestDBCC];

SELECT * FROM #ShrinkFileResults;

This worked for me on both SQL Server 2012 SP3 and SQL Server 2016 RTM.

NOTES

• Using OPENQUERY does not work. Initially you get a "meta-data discovery" error which can be resolved by wrapping the DBCC call in another EXEC that uses the WITH RESULT SETS option. But then you get an error about "OPENQUERY cannot process object as it either does not exist or you don't have permission to it". But if you change the logical name of the file to shrink, then you get an error from DBCC stating that it cannot find a file of that name, along with a secondary error from OPENQUERY stating that the call did not return any result sets, all of which indicates that with the correct file name it is get farther.

• This operation can also be rather simply accomplished via SQLCLR. One can create a UDF / Scalar function, in fact, using an external connection (not a Context Connection), which would allow for being called in a set-based operation in the SELECT list. That would also only take a few lines of code, and minimize security risk when compared to enabling xp_cmdshell or creating a Linked Server.