While indexes can speed up execution of queries several fold as they can make the querying process faster, there is overhead associated with them. They consume additional disk space and require additional time to update themselves whenever data is updated, deleted or appended in a table. Also when you perform any data modification operations (INSERT, UPDATE, or DELETE statements) index fragmentation may occur and the information in the index can get scattered in the database. Fragmented index data can cause SQL Server to perform unnecessary data reads and switching across different pages, so query performance against a heavily fragmented table can be very poor. In this article I am going to write about fragmentation and different queries to determine the level of fragmentation.
When indexes are first built, little or no fragmentation should exist. Over time, as data is inserted, updated, and deleted, fragmentation levels on the underlying indexes may begin to rise. So let’s see how it happens.
When a page of data fills to 100 percent and more data must be added to it, a page split occurs. To make room for the new incoming data, SQL Server moves half of the data from the full page to a new page. The new page that is created is created after all the pages in the database. Therefore, instead of going right from one page to the next when looking for data, SQL Server has to go from one page to another page somewhere else in the database looking for the next page it needs. This is called index fragmentation.
There are basically two types of fragmentation:
- External fragmentation – External, a.k.a logical, fragmentation occurs when an index leaf page is not in logical order, in other words it occurs when the logical ordering of the index does not match the physical ordering of the index. This causes SQL Server to perform extra work to return ordered results. For the most part, external fragmentation isn’t too big of a deal for specific searches that return very few records or queries that return result sets that do not need to be ordered.
- Internal fragmentation – Internal fragmentation occurs when there is too much free space in the index pages. Typically, some free space is desirable, especially when the index is created or rebuilt. You can specify the Fill Factor setting when the index is created or rebuilt to indicate a percentage of how full the index pages are when created. If the index pages are too fragmented, it will cause queries to take longer (because of the extra reads required to find the dataset) and cause your indexes to grow larger than necessary. If no space is available in the index data pages, data changes (primarily inserts) will cause page splits as discussed above, which also require additional system resources to perform.
Here are some code snippets for your references. To get the complete script sample, please click the download button at the beginning of this page.
SELECT OBJECT_NAME(ind.OBJECT_ID) AS TableName, ind.name AS IndexName, indexstats.index_type_desc AS IndexType, indexstats.avg_fragmentation_in_percent FROM sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, NULL) indexstats INNER JOIN sys.indexes ind ON ind.object_id = indexstats.object_id AND ind.index_id = indexstats.index_id WHERE indexstats.avg_fragmentation_in_percent > 60 ORDER BY indexstats.avg_fragmentation_in_percent DESC