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Maximum size of index and data pages
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SQL Server 2005 allows a maximum of 8,060 bytes on a single data or index page. The number of data or index rows allowed on a single page is the result of 8,060 divided by the size of the individual data or index row. For example, if you have an index containing 800 bytes of column key values, SQL Server can store only 10 index rows per index page.
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When SQL Server needs to place additional rows of data on an index page or data page and that page cannot accommodate the additional row, a page split occurs a new page is added, and SQL Server splits the rows of data or index information on the original page between the original page and the new page. Page splits maintain the logical order of the rows in the index key; SQL Server knows which page the next index key is located on. But page splits do not maintain the physical ordering of the page; the new page usually won t be physically adjacent to the original page on the disk. When pages are not in physical order, there is external fragmentation. External fragmentation is always undesirable in an index, whereas a small amount of internal fragmentation can be desirable in highly transactional databases because it prevents large numbers of page splits. However, large-scale internal and external fragmentations adversely affect the performance of retrieving data. In the case of internal fragmentation, rows are distributed sparsely across a large number of pages, increasing the number of disk input/output (I/O) operations that SQL Server must perform to read the index pages into memory as well as the number of logical reads it must perform to read multiple index rows from memory.
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Lesson 1: Managing Index Fragmentation
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External fragmentation causes a noncontiguous sequence of index pages on the disk, with new leaf pages far from the original leaf pages and their physical ordering different from their logical ordering. Consequently, when SQL Server performs a range scan on an index that has external fragmentation, it needs to switch between corresponding extents more than what would be ideal. Also, a range scan on an externally fragmented index will not benefit from read-ahead operations performed on the disk. If the pages are arranged contiguously, SQL Server can perform a read-ahead operation to read pages in advance without much head movement.
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For information about creating indexes and the types of indexes you can create, see 4, Creating Indexes, in this book and the Indexes topic in SQL Server 2005 Books Online. SQL Server 2005 Books Online is installed as part of SQL Server 2005. Updates for SQL Server 2005 Books Online are available for download at www.microsoft.com/technet/prodtechnol/sql/2005/downloads/ books.mspx.
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Identifying Index Fragmentation
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To help you determine the amount of external and internal fragmentation on indexes that are contained within a database, SQL Server 2005 provides the sys.dm_db_index_physical_stats dynamic management function (DMF). By issuing a simple SELECT statement against this index DMF, you can check the values contained in the avg_fragmentation_in_percent and avg_page_space_used_in_percent columns, as the following sample query shows, and determine whether your indexes are suffering from fragmentation:
--Determine index fragmentation for all tables in the AdventureWorks database SELECT OBJECT_NAME(dt.object_id), si.name, dt.avg_fragmentation_in_percent, dt.avg_page_space_used_in_percent FROM (SELECT object_id, index_id, avg_fragmentation_in_percent, avg_page_space_used_in_percent FROM sys.dm_db_index_physical_stats (DB_ID( AdventureWorks ), NULL, NULL, NULL, DETAILED ) WHERE index_id <> 0) as dt --does not return information about heaps INNER JOIN sys.indexes si ON si.object_id = dt.object_id AND si.index_id = dt.index_id
DBAs should review the value in the avg_fragmentation_in_percent column to determine whether the index contains external fragmentation. External fragmentation is indicated when this value exceeds 10.
12
Using Transact-SQL to Manage Databases
DBAs should review the value in the avg_page_space_used_in_percent column to determine whether the index contains internal fragmentation. Internal fragmentation is indicated when this value falls below 75.
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