The histogram and density values for an index are not maintained as data rows are added and deleted. The database owner must issue an update statistics command to ensure that statistics are current. Run update statistics:
After deleting or inserting rows that change the skew of key values in the index
After adding rows to a table whose rows were previously deleted with truncate table
After updating values in index columns
Run update statistics after inserts to any index that includes an IDENTITY column or any increasing key value. Date columns often have regularly increasing keys.
Running update statistics on these types of indexes is especially important if the IDENTITY column or other increasing key is the leading column in the index. After a number of rows have been inserted past the last key in the table when the index was created, all that the optimizer can tell is that the search value lies beyond the last row in the distribution page.
It cannot accurately determine how many rows match a given value.
Failure to update statistics can severely hurt performance.
See Chapter 3, “Using Statistics to Improve Performance,” in the Performance and Tuning: Monitoring and Analyzing for Performance book for more information.
