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Tamarick Hill

July, the 31 Days of SQL Server DMO’s – Day 22 (sys.dm_db_index_physical_stats)

The sys.dm_db_index_physical_stats Dynamic Management Function is used to return information about the fragmentation levels, page counts, depth, number of levels, record counts, etc. about the indexes on your database instance. One row is returned for each level in a given index, which we will discuss more later. The function takes a total of 5 input parameters which are (1) database_id, (2) object_id, (3) index_id, (4) partition_number, and (5) the mode of the scan level that you would like to run. Let’s use this function with our AdventureWorks2012 database to better illustrate the information it provides.

SELECT * FROM sys.dm_db_index_physical_stats(db_id('AdventureWorks2012'), NULL, NULL, NULL, NULL)

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As you can see from the result set, there is a lot of beneficial information returned from this DMF. The first couple of columns in the result set (database_id, object_id, index_id, partition_number, index_type_desc, alloc_unit_type_desc) are either self-explanatory or have been explained in our previous blog sessions so I will not go into detail about these at this time. The next column in the result set is the index_depth which represents how deep the index goes. For example, If we have a large index that contains 1 root page, 3 intermediate levels, and 1 leaf level, our index depth would be 5. The next column is the index_level which refers to what level (of the depth) a particular row is referring to. Next is probably one of the most beneficial columns in this result set, which is the avg_fragmentation_in_percent. This column shows you how fragmented a particular level of an index may be. Many people use this column within their index maintenance jobs to dynamically determine whether they should do REORG’s or full REBUILD’s of a given index. The fragment count represents the number of fragments in a leaf level while the avg_fragment_size_in_pages represents the number of pages in a fragment. The page_count column tells you how many pages are in a particular index level.

From my result set above, you see the the remaining columns all have NULL values. This is because I did not specify a ‘mode’ in my query and as a result it used the ‘LIMITED’ mode by default. The LIMITED mode is meant to be lightweight so it does collect information for every column in the result set. I will re-run my query again using the ‘DETAILED’ mode and you will see we now have results for these rows.

SELECT * FROM sys.dm_db_index_physical_stats(db_id('AdventureWorks2012'), NULL, NULL, NULL, ‘DETAILED’)

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From the remaining columns, you see we get even more detailed information such as how many records are in a particular index level (record_count). We have a column for ghost_record_count which represents the number of records that have been marked for deletion, but have not physically been removed by the background ghost cleanup process. We later see information on the MIN, MAX, and AVG record size in bytes. The forwarded_record_count column refers to records that have been updated and now no longer fit within the row on the page anymore and thus have to be moved. A forwarded record is left in the original location with a pointer to the new location. The last column in the result set is the compressed_page_count column which tells you how many pages in your index have been compressed.

This is a very powerful DMF that returns good information about the current indexes in your system. However, based on the mode you select, it could be a very resource intensive function so be careful with how you use it.

For more information on this Dynamic Management Function, please see the below Books Online link:

http://msdn.microsoft.com/en-us/library/ms188917.aspx

Follow me on Twitter @PrimeTimeDBA

Published Monday, July 22, 2013 10:16 AM by Tamarick Hill
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