SQL Server statistics

Statistics are information about the distribution of data in a column, maintained internally by the database engine.

Rules and behaviour with regard to statistics, when automatic creation/updating is turned on:

• Statistics are automatically created when creating an index, but only for the first column of the (composite) index.
• Statistics can be created manually on any column or combination of columns.
• Statistics are automatically created/updated when the query optimizer finds need for them.
• Statistics are not automatically created/updated when the server is too busy, unless the "update statistics asynchronously" option is turned on, because then the are created in parallel.
• Asynchronously created/updated statistics have effect the next time the (causing) query is run.

Links:
http://www.sql-server-performance.com/tips/update_statistics_p1.aspx
http://blogs.technet.com/b/rob/archive/2008/05/16/sql-server-statistics.aspx
http://technet.microsoft.com/nl-nl/library/cc966419(en-us).aspx