Here is a quick demonstration that shows you what can happen when you use defaults that are much shorter than the value that is updated from an insert trigger. The ModifiedBy column has a default of “ but in the trigger it gets updated to ‘SomeapplicationName used by ’ +SUSER_NAME(). Ideally you want the default to also be ‘SomeapplicationName used by ’ +SUSER_NAME()

Let’s take a look, first create the following table

CREATE TABLE TestFrag (
	id INT NOT NULL IDENTITY PRIMARY KEY, 
	SomeData UNIQUEIDENTIFIER DEFAULT newsequentialid(),
	Somedate datetime DEFAULT GETDATE(),
	SomeOtherData CHAR(200) DEFAULT 'bla',
	ModifiedBy varchar(100) DEFAULT '')

Add a trigger on that table

CREATE TRIGGER [dbo].[Tr_TestFrag] ON [dbo].[TestFrag] FOR Insert 
AS
UPDATE t
        SET    ModifiedBy = 'SomeapplicationName used by ' +SUSER_NAME()
        FROM   TestFrag t
        JOIN inserted i ON t.id = i.id
       
GO

Now add another identical table, the only difference is that the default matches what gets updated from within the trigger

CREATE TABLE TestFrag2 (
	id INT NOT NULL IDENTITY PRIMARY KEY, 
	SomeData UNIQUEIDENTIFIER DEFAULT newsequentialid(),
	Somedate datetime DEFAULT GETDATE(),
	SomeOtherData CHAR(200) DEFAULT 'bla',
	ModifiedBy varchar(100) DEFAULT 'SomeapplicationName used by ' +SUSER_NAME())

Here is the other trigger

CREATE TRIGGER [dbo].[Tr_TestFrag2] ON [dbo].[TestFrag2] FOR Insert 
AS
UPDATE t
        SET    ModifiedBy = 'SomeapplicationName used by ' +SUSER_NAME()
        FROM   TestFrag2 t
        JOIN inserted i ON t.id = i.id
       
GO

Now let’s pump 100000 rows into both tables

INSERT TestFrag(Somedate)
SELECT TOP 100000 GETDATE() 
FROM sysobjects s1
CROSS JOIN sysobjects s2
CROSS JOIN sysobjects s3

INSERT TestFrag2(Somedate)
SELECT TOP 100000 GETDATE() 
FROM sysobjects s1
CROSS JOIN sysobjects s2
CROSS JOIN sysobjects s3

Run this to just verify that we have the same data

SELECT TOP 10 * FROM TestFrag
SELECT TOP 10 * FROM TestFrag2

Now let’s look how much space each table is using

EXEC sp_spaceused 'TestFrag'
EXEC sp_spaceused 'TestFrag2'

name       rows        reserved    data        index_size  unused
TestFrag    100000      47240 KB    47064 KB    104 KB      72 KB
TestFrag2   100000      28744 KB    28576 KB    112 KB      56 KB

See that, the table with the same default as in the trigger is using a lot less data?

Now let’s see how bad the fragmentation is. Here is the 2005 and up version

SELECT  
    OBJECT_NAME(i.OBJECT_ID) AS TableName
    ,indexstats.index_id
    ,index_type_desc
    ,avg_fragmentation_in_percent
    ,page_count
    ,record_count
    ,fill_factor
FROM    
sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, 'DETAILED') indexstats
INNER JOIN sys.indexes i ON i.OBJECT_ID = indexstats.OBJECT_ID
AND i.index_id = indexstats.index_id
WHERE OBJECT_NAME(i.OBJECT_ID) in ('TestFrag','TestFrag2')

Yikes, do you see that, one table is completely fragmented.

Here is the 2000 version by using showcontig

DBCC SHOWCONTIG scanning 'TestFrag' table...
Table: 'TestFrag' (933578364); index ID: 1, database ID: 14
TABLE level scan performed.
- Pages Scanned................................: 5883
- Extents Scanned..............................: 738
- Extent Switches..............................: 5879
- Avg. Pages per Extent........................: 8.0
- Scan Density [Best Count:Actual Count].......: 12.52% [736:5880]
- Logical Scan Fragmentation ..................: 99.42%
- Extent Scan Fragmentation ...................: 0.27%
- Avg. Bytes Free per Page.....................: 3234.5
- Avg. Page Density (full).....................: 60.04%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.


DBCC SHOWCONTIG scanning 'TestFrag2' table...
Table: 'TestFrag2' (1205579333); index ID: 1, database ID: 14
TABLE level scan performed.
- Pages Scanned................................: 3572
- Extents Scanned..............................: 450
- Extent Switches..............................: 449
- Avg. Pages per Extent........................: 7.9
- Scan Density [Best Count:Actual Count].......: 99.33% [447:450]
- Logical Scan Fragmentation ..................: 0.34%
- Extent Scan Fragmentation ...................: 0.44%
- Avg. Bytes Free per Page.....................: 89.3
- Avg. Page Density (full).....................: 98.90%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

As you can see you will get horrible page splits if your trigger expands the row every time an insert happens.

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