Microsoft® SQL Server 2008 R2 Unleashed

Generating and Maintaining Index and Column Statistics

At this point, you might ask, “How do the index statistics get created?” and “How are they maintained?” The index statistics are first created when you create the index on a table that already contains data rows or when you run the UPDATE STATISTICS command. Index statistics can also be automatically updated by SQL Server. SQL Server can be configured to constantly monitor the update activity on the indexed key values in a database and update the statistics through an internal process, when appropriate.

Auto-Update Statistics

To automatically update statistics, an internal SQL Server process monitors the updates to a table’s columns to determine when statistics should be updated. SQL Server ...