Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Page Splits
When a page becomes full, it splits. This means more than just a new page being allocated — it also
means that approximately half the data from the existing page is moved to the new page.
The exception to this process is when a clustered index is in use. If there is a clustered index and the next
inserted row would be physically located as the last record in the table, then a new page is created, and
the new row is added to the new page without relocating any of the existing data. We will see much
more on page splits as we investigate indexes.
Rows
You have heard much about “row level locking,” so it shouldn’t be a surprise to hear this term. Rows
can be up to 8KB.
In addition to the limit of 8,060 characters, there is also a maximum of 1,024 standard (non-sparse)
columns. In practice, you’ll find it very unusual to run into a situation where you run out of columns
before you run into the 8,060 character limit. 1,024 gives you an average column width of just under
8 bytes. For most uses, you’ll easily exceed that average (and therefore exceed the 8,060 characters before
the 1,024 columns). The exception to this tends to be in measurement and statistical information — where
you have a large number of different things that you are storing numeric samples of. Still, even those
applications will find it a rare day when they bump into the 1,024-column-count limit. When you do,
you can explore the notion of sparse columns, so let’s look at that.
Sparse Columns
Chapter 9: SQL Server Storage and Index Structures
Sparse columns, in terms of a special data structure, is new with SQL Server 2008. These are meant to deal
with the recurring scenario where you have columns that you essentially just need “sometimes.” That is,
they are going to be null a high percentage of the time. There are many scenarios where, if you bump
into a few of these kinds of columns, you tend to bump into a ton of them. Using sparse columns, you
can increase the total number of allowed columns in a single table to 30,000.
Internally, the data from columns marked as being sparsely populated is embedded within a single
column — allowing a way to break the former limitation of 1,024 columns without major architectural
changes.
Image, text, ntext, geography, geometry, timestamp, and all user-defined data types are prohibited
from being marked as a sparse column.
While sparse columns are handled natively by newer versions of the SQL Native
Client, other forms of data access will have varying behavior when accessing sparse
columns. The sparse property of a column will be transparent when selecting a column
by name, but when selecting it using a “*” in the select list, different client
access methods will vary between supplying the sparse columns as a unified XML
column vs. not showing those columns at all. You’ll want to upgrade your client
libraries as soon as reasonably possible.
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