Skip to main content

Reclaiming the table space after dropping a column - [With Clustered Index]

If we drop a column it gets dropped but the space which it was occupying stays as it is! In this article we would see the way to reclaim the space for a table which has a clustered Index.

Create a table with clustered index in it:

Create Table tblDemoTable (
[Sno] int primary key clustered,
[Remarks] char(5000) not null
)
Go

Pump-in some data into the newly created table:

Set nocount on
Declare @intRecNum int
Set @intRecNum = 1

While @intRecNum <= 15000

Begin
Insert tblDemoTable (Sno, Remarks ) Values (@intRecNum, convert(varchar,getdate(),109))
Set @intRecNum = @intRecNum + 1
End

If it's SQL 2000 or earlier:

DBCC SHOWCONTIG ('dbo.tblDemoTable') -- Displays fragmentation information for the data and indexes of the specified table
Go

Output:

DBCC SHOWCONTIG scanning 'tblDemoTable' table...
Table: 'tblDemoTable' (1717581157); index ID: 1, database ID: 9
TABLE level scan performed.
- Pages Scanned................................: 80
- Extents Scanned..............................: 12
- Extent Switches..............................: 11
- Avg. Pages per Extent........................: 6.7
- Scan Density [Best Count:Actual Count].......: 83.33% [10:12]
- Logical Scan Fragmentation ..................: 3.75%
- Extent Scan Fragmentation ...................: 8.33%
- Avg. Bytes Free per Page.....................: 33.7
- Avg. Page Density (full).....................: 99.58%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

If you are using SQL 2005:

As that DBCC feature would be removed in the future version of SQL Server I would suggest the following code snippet instead of DBCC SHOWCONTIG.

Declare @object_id int;
Set @object_id = Object_ID(N'Testbed.dbo.tblDemoTable');

Begin
Select index_type_desc, index_depth, index_level,
avg_fragmentation_in_percent, avg_fragment_size_in_pages,
page_count, avg_page_space_used_in_percent, record_count
from sys.dm_db_index_physical_stats (Db_id(), @object_id, NULL, NULL , 'Detailed');
End
Go

Output:


Drop the column 'Remarks' from the table:

Alter table tblDemoTable drop column Remarks
Go


Now try out DBCC SHOWCONTIG or sys.dm_db_index_physical_stats as explained previously and verify that the details haven't changed a bit :)

Solution:

DBCC DBREINDEX ( 'dbo.tblDemoTable' )
Go

Now try out either the SHOWCONTIG or dm_db_index_physical_stats and see that you have reclaimed the space successfully.

DBCC SHOWCONTIG ('dbo.tblDemoTable')
Go

Output:

DBCC SHOWCONTIG scanning 'tblDemoTable' table...
Table: 'tblDemoTable' (1717581157); index ID: 1, database ID: 9
TABLE level scan performed.
- Pages Scanned................................: 25
- Extents Scanned..............................: 5
- Extent Switches..............................: 4
- Avg. Pages per Extent........................: 5.0
- Scan Density [Best Count:Actual Count].......: 80.00% [4:5]
- Logical Scan Fragmentation ..................: 8.00%
- Extent Scan Fragmentation ...................: 20.00%
- Avg. Bytes Free per Page.....................: 296.0
- Avg. Page Density (full).....................: 96.34%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

May be this is one another good example of why we need to have clustered index on a table :)

Similarly if you have run this DMV sys.dm_db_index_physical_stats then the output would be this:



BTW, in SQL 2005 though DBCC DBREINDEX would work, its better to start practicing ALTER INDEX syntax.

In the next post we would see how to reclaim the space in a table which doesn't have clustered index in it.

Related Article: Reclaim Unused Table space. This article was written couple of years back and was tested with SQL 2000 at that time.

Comments

sasi said…
dear Sir,
is it advisiable to use dbccreindex for reclaiming the table space.plz need help.

with regards,
R.Sasi

Popular posts from this blog

Registry manipulation from SQL

Registry Manupulation from SQL Server is pretty easy. There are 4 extended stored procedure in SQL Server 2000 for the purpose of manupulating the server registry. They are: 1) xp_regwrite 2) xp_regread 3) xp_regdeletekey 4) xp_regdeletevalue Let us see each one of them in detail! About xp_regwrite This extended stored procedure helps us to create data item in the (server’s) registry and we could also create a new key. Usage: We must specify the root key with the @rootkey parameter and an individual key with the @key parameter. Please note that if the key doesn’t exist (without any warnnig) it would be created in the registry. The @value_name parameter designates the data item and the @type the type of the data item. Valid data item types include REG_SZ and REG_DWORD . The last parameter is the @value parameter, which assigns a value to the data item. Let us now see an example which would add a new key called " TestKey ", and a new data item under it called TestKeyValue :

Screen scraping using XmlHttp and Vbscript ...

I wrote a small program for screen scraping any sites using XmlHttp object and VBScript. I know I haven't done any rocket science :) still I thought of sharing the code with you all. XmlHttp -- E x tensible M arkup L anguage H ypertext T ransfer P rotocol An advantage is that - the XmlHttp object queries the server and retrieve the latest information without reloading the page. Source code: < html > < head > < script language ="vbscript"> Dim objXmlHttp Set objXmlHttp = CreateObject("Msxml2.XMLHttp") Function ScreenScrapping() URL == "UR site URL comes here" objXmlHttp.Open "POST", url, False objXmlHttp.onreadystatechange = getref("HandleStateChange") objXmlHttp.Send End Function Function HandleStateChange() If (ObjXmlHttp.readyState = 4) Then msgbox "Screenscrapping completed .." divShowContent.innerHtml = objXmlHttp.responseText End If End Function </ script > < head > < body > &l

Script table as - ALTER TO is greyed out - SQL SERVER

One of my office colleague recently asked me why we are not able to generate ALTER Table script from SSMS. If we right click on the table and choose "Script Table As"  ALTER To option would be disabled or Greyed out. Is it a bug? No it isn't a bug. ALTER To is there to be used for generating modified script of Stored Procedure, Functions, Views, Triggers etc., and NOT for Tables. For generating ALTER Table script there is an work around. Right click on the table, choose "Modify" and enter into the design mode. Make what ever changes you want to make and WITHOUT saving it right click anywhere on the top half of the window (above Column properties) and choose "Generate Change Script". Please be advised that SQL Server would drop actually create a new table with modifications, move the data from the old table into it and then drop the old table. Sounds simple but assume you have a very large table for which you want to do this! Then it woul