Performance Tuning with SQL Server Dynamic Management Views

Chapter 5: Indexing Strategy and Maintenance
Fragmentation statistics (index_physical_stats)
The sys.dm_db_index_physical_stats DMF provides invaluable information
regarding the state of your index partitions, including type of index, index depth, and
degree of fragmentation. An index in this context can mean several things: a clustered
index, heap, index, or a partition of either of these. These physical stats all serve to tell
you of the condition of an index. Even the most potentially beneficial index in terms of
optimizing query execution can become useless if it is not maintained properly.
Data stored in this DMF is cumulative but refreshed when the server is restarted or
when the index is dropped and recreated. Statistics live on when the index is rebuilt or
reorganized, and even when it is disabled and rebuilt.
Like the operational_stats DMF, the physical_stats DMF accepts the
database_id, object_id, index_id, and partition_number as parameters, in
order to identify the object (heap, index or partition) in question, and returns detailed
"physical stats" for each partition. All the parameters can be NULL or DEFAULT if you
want to return all rows, in which case the DMF will return a row for every partition in
every database. It also accepts a mode parameter, which determines the thoroughness
with which the statistics are collected. Possible values are:
• LIMITED is the default and the least costly, but it also will leave a lot of the columns as
NULL; LIMITED will not scan the leaf level of the indexes and the data pages of heaps
are not scanned
• SAMPLED mode returns statistics based only upon a 1% sample of all pages of the
indexes and heaps in the scope of the function call; if any page or heap within
the scope of the function call has less than 10,000 pages, then DETAILED is
automatically used
• DETAILED provides the most complete result set from this function of the three, but
can require ample resources; it scans all pages; it returns all statistics.
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