Reclaiming the table space after dropping a column [without 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 non-clustered Index.

Create a table with non-clustered index in it:

Create Table tblDemoTable_nonclustered (
[Sno] int primary key nonclustered,
[Remarks] varchar(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_nonclustered (Sno, Remarks ) Values (@intRecNum, convert(varchar,getdate(),109)) 
Set @intRecNum = @intRecNum + 1 
End 

Check the fragmentation info before dropping the column:

DBCC SHOWCONTIG ('dbo.tblDemoTable_nonclustered')
GO

Output:

DBCC SHOWCONTIG scanning 'tblDemoTable_nonclustered' table...
Table: 'tblDemoTable_nonclustered' (1781581385); index ID: 0, database ID: 9
TABLE level scan performed.
- Pages Scanned................................: 84
- Extents Scanned..............................: 12
- Extent Switches..............................: 11
- Avg. Pages per Extent........................: 7.0
- Scan Density [Best Count:Actual Count].......: 91.67% [11:12]
- Extent Scan Fragmentation ...................: 41.67%
- Avg. Bytes Free per Page.....................: 417.4
- Avg. Page Density (full).....................: 94.84%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

Drop the Remarks column and reindex the table:

Alter table tblDemoTable_nonclustered drop column Remarks
go

DBCC DBREINDEX ( 'dbo.tblDemoTable_nonclustered' )
Go

Now check the Fragmentation info:

DBCC SHOWCONTIG ('dbo.tblDemoTable_nonclustered')
GO

Output:

DBCC SHOWCONTIG scanning 'tblDemoTable_nonclustered' table...
Table: 'tblDemoTable_nonclustered' (1781581385); index ID: 0, database ID: 9
TABLE level scan performed.
- Pages Scanned................................: 84
- Extents Scanned..............................: 12
- Extent Switches..............................: 11
- Avg. Pages per Extent........................: 7.0
- Scan Density [Best Count:Actual Count].......: 91.67% [11:12]
- Extent Scan Fragmentation ...................: 41.67%
- Avg. Bytes Free per Page.....................: 417.4
- Avg. Page Density (full).....................: 94.84%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

If you see there won't be any difference or rather the space hasn't reclaimed yet. Here, DBCC DBREINDEX won't work as nonclustered index are stored in heap. i.e., Heaps are tables that have no clustered index.

Solution:

Select * into #temp from tblDemoTable_nonclustered
Go

Truncate table tblDemoTable_nonclustered
Go

Insert into tblDemoTable_nonclustered select * from #temp
Go

Now check the fragmentation info to see that we have actually reclaimed the space!

DBCC SHOWCONTIG scanning 'tblDemoTable_nonclustered' table...
Table: 'tblDemoTable_nonclustered' (1781581385); index ID: 0, database ID: 9
TABLE level scan performed.
- Pages Scanned................................: 25
- Extents Scanned..............................: 7
- Extent Switches..............................: 6
- Avg. Pages per Extent........................: 3.6
- Scan Density [Best Count:Actual Count].......: 57.14% [4:7]
- Extent Scan Fragmentation ...................: 57.14%
- 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.

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.