Monday, October 01, 2007

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.

Hope this helps!

3 comments:

Bharath Pammi said...

Select * into #temp from tblDemoTable_nonclustered
Go

Truncate table tblDemoTable_nonclustered
Go

Insert into tblDemoTable_nonclustered select * from #temp
Go

So the table space is reclaimed, when we truncate the table and repopulate the data.Is my understanding correct..?

Vadivel said...

Yes if your tbl doesn't have a clustered index then it means you have a table heap.

So dropping a column doesn't recover you the space. You need to copy the data out, truncate the table and then copy back the data into it.

So I always suggest you to use 'Clustered Index' in you table design.

Andre said...

And if my table has both indexes ?