Statistics Used by the Query Optimizer in Microsoft SQL Server 2005 (Microsoft TechNet SQL Server TechCenter)
Microsoft SQL Server 2005 collects statistical information about indexes and column data stored in the database. These statistics are used by the SQL Server query optimizer to choose the most efficient plan for retrieving or updating data. This paper describes what data is collected, where it is stored, and which commands create, update, and delete statistics. By default, SQL Server 2005 also creates and updates statistics automatically, when such an operation is considered to be useful. This paper also outlines how these defaults can be changed on different levels (column, table, and database).