Introduction
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 and pump in some dummy data into it
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
Check the output for yourself and then proceed further.
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 run DMV sys.dm_db_index_physical_stats then check the output for yourself.
BTW, in SQL 2005 though DBCC DBREINDEX would work, its better to start practicing ALTER INDEX syntax. In the next article we would see how to reclaim the space in a table which doesn't have clustered index in it.
|
No responses found. Be the first to respond and make money from revenue sharing program.
|