Maximum Row Size in SQL Server 2005

,

I've been using SQL 2005 on a part time basis for a while now (those SQL

2000 servers work pretty good you know!) and I'm still intrigued at the things I

find that have changed the rules of the game. It's pretty commonly known that

SQL pages are 8k, and that the max row size is 8060 bytes (though Steve Jones

proved that isn't always the case in

What is

the Maximum Page Size in SQL Server 2000? But in SQL 2005, the max row size

of 8060 bytes is even less true than it was before!

Assuming I'm not the only one that still have a SQL2K instance, create a test

table as follows:

CREATE TABLE [dbo].[TestTable](
[LargeColumn1] [varchar](8000) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
[LargeColumn2] [varchar](8000) COLLATE SQL_Latin1_General_CP1_CI_AS NULL
) ON [PRIMARY]
GO

You should get the following warning:

Warning: The table 'TestTable' has been created but its maximum row size

(16025) exceeds the maximum number of bytes per row (8060). INSERT or UPDATE of

a row in this table will fail if the resulting row length exceeds 8060 bytes.

The problem is that this table could work fine for a day, a week, or a

lifetime, and then boom, you get bitten by your bad table design. Just to prove

that really happens, let's inject a row and then make some changes:

insert into Testtable (LargeColumn1, LargeColumn2) values ('Blah1', 'Blah2')
update testtable set largecolumn1 = replicate ('0123456789', 800)

At this point LargeColumn1 is 8000 bytes, leaving us the 60 or so bytes for

column LargeColumn2. Let's see what happens if we try to update it with 100

characters:

update testtable set largecolumn2 = replicate ('0123456789', 10)

We get this lovely error:

Msg 511, Level 16, State 1, Line 1

Cannot create a row of size 8113 which is greater than the allowable maximum

of 8060.

The statement has been terminated.

In fact, the most we can stuff into LargeColumn2 is an additional 47 bytes.

Now suppose we try our scenario on a SQL 2005 instance, what do you think might

happen? The first thing is you get NO WARNING that you're exceeding the 8060

byte when you run the create table statement . Why? Because it's not a real

limit anymore! Here is an except from BOL (search on row-overflow to find the

entire detail):

Surpassing the 8,060-byte row-size limit might affect performance because

SQL Server 2005 Database Engine still maintains a limit of 8 KB per page. When a

combination of varchar, nvarchar, varbinary, sql_variant,

or CLR user-defined type columns exceeds this limit, the Database Engine moves

the record column with the largest width to another page in the

ROW_OVERFLOW_DATA allocation unit, while maintaining a 24-byte pointer on the

original page. Moving large records to another page occurs dynamically as

records are lengthened based on update operations. Update operations that

shorten records may cause records to be moved back to the original page in the IN_ROW_DATA allocation unit. Also, querying and performing other select

operations, such as sorts or joins on large records that contain row-overflow

data slows processing time, because these records are processed synchronously

instead of asynchronously.

We can fully populate both columns with no error message:

insert into Testtable (LargeColumn1, LargeColumn2) values ('Blah1', 'Blah2')
update testtable set largecolumn1 = replicate ('0123456789', 800)
update testtable set largecolumn2 = replicate ('0123456789', 800)

This new behavior affects the entire record

rather than a single column, and in the worst case you could have parts of columns

bouncing back and forth between in row and out of row storage. Here's another

interesting twist; setting the compatibility level to 70 or 80 on a SQL 2005

instance does not change the behavior - so you could update a SQL 2000 server,

set SQL2K compatibility, and still use the new behavior. One side affect of that

is that if you had designed a table in SQL 2000 where the total width exceeded

the maximum and always worried that you might get burned someday this could save

you!

I haven't decided if I like the new behavior or not. My Zen side says that

there is a time and place for all options, but the more practical side says that

sometimes it's good to have some solid rules to work within. One thing I do

dislike is that the new behavior is silent. If you've been a DBA for a while you

probably rely on that warning message to keep you from creating super wide

tables by accident, but now you'll have to work harder and total up the bytes. I

think a new version of the warning message would have been nice, something like

this:

Feature Change Warning: The table 'TestTable' has been created but its maximum row size

(16025) exceeds the maximum number of bytes per row (8060). On instances of SQL

2000 and earlier INSERT or UPDATE of

a row in this table will fail if the resulting row length exceeds 8060 bytes. On

SQL 2005 instances using any compatibility level there are cases where it is

possible to exceed the 8060 byte restriction. Please see row-overflow in BOL for

more information.

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