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SQL Server can automatically maintain statistics, periodically refreshing them based on its tracking of data modifications.

However, for some tables, such as those subject to significant changes in distribution, or those with skewed values, it’s possible that SQL Server’s automatic statistics update will be inadequate to maintain consistently high levels of query performance.

In contrast, if you have invalid or out-of-date statistics, it can lead the SQL Server engine to take the wrong path to the data, and taking the wrong path means that an index scan is made when an index seek would have been appropriate or a seek is performed against the wrong index.

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During the PASS Summit in November I presented a session on Demystifying Database Statistics (if you attended Summit but missed it, you can stream it from here).The statistics on an index are created with the index.are metadata about the data within a table and they're created and updated on a column-by-column basis, not on the entire table.Bad or missing statistics leads to poor choices by the optimizer: The result is horrific performance.Recently, for example, I watched a query go from taking hours to taking seconds just because we got a good set of statistics on the data.I just created the table, so the Last Updated date is current, and I have not made any changes so the modification count is 0.

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