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

Chapter 9: SQL Server Storage and Index Structures
That being said, database fragmentation does have its good side — OLTP systems positively love fragmentation.
Any guesses as to why? Page splits. Pages that don’t have much data in them can have data
inserted with little or no risk of page splits.
So, high fragmentation equates to poor read performance, but it also equates to excellent insert performance.
As you might expect, this also means that OLAP systems really don’t like fragmentation.
Identifying Fragmentation vs. Likelihood of Page Splits
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SQL Server provides a special metadata function called sys.dm_db_index_physical_stats that is
particularly useful in helping us identify just how full the pages and extents in our database are. We can
then use that information to make some decisions about what we want to do to maintain our database.
sys.dm_db_index_physical_stats is what is called a table valued function (we’ll learn more about
these in Chapter 13). In short, this means that while it’s a function, you can use it much as you would a
table, which means that you can actually stick WHERE conditions on it and other similar constructs. The
syntax for the function itself is pretty simple:
sys.dm_db_index_physical_stats (
{ | NULL | 0 | DEFAULT }
, { | NULL | 0 | DEFAULT }
, { | NULL | 0 | -1 | DEFAULT }
, { | NULL | 0 | DEFAULT }
, { | NULL | DEFAULT }
)
Note that it is demanding the rather non-intuitive ids as input values rather than the logical names that
you might be more used to. Fortunately, SQL Server gives us a number of functions that work on returning
the proper id for a given object on the server or in the database.
As an example of how to use this, let’s get the all information index information from the Sales
.SalesOrderDetail table. Since we’ve said we want all the indexes in the table, we just need the id for
the table — not any individual indexes. Since the function is table valued, we need to write a query to
return the results from the function. For example:
DECLARE @db_id SMALLINT;
DECLARE @object_id INT;
SET @db_id = DB_ID(N'AdventureWorks2008');
SET @object_id = OBJECT_ID(N'AdventureWorks2008.Sales.SalesOrderDetail');
SELECT database_id, object_id, index_id, index_depth, avg_fragmentation_in_percent,
page_count
FROM sys.dm_db_index_physical_stats(@db_id,@object_id,NULL,NULL,NULL);