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.The database engine employs statistics when generating execution plans that are used to access data stored within the database.Typically, both should be left enabled for databases.

For that reason, having accurate and up-to-date statistics is crucial.

In contrast, if you have invalid or out-of-date statistics, it can lead the SQL Server engine to take the wrong path to the data, and taking the wrong path means that an index scan is made when an index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

During the first pass the rows are updated using the primary key value, whilst the rowid is used in the second pass. The next section will show how the order of logical expressions and branching structures can affect the performance of code.

Short-circuit evaluations and ordering logic When conditional logic contains several expressions separated by the AND or OR operators it is sometimes possible to determine the final result without evaluating all the expressions.

Manually updating statistics There are two ways to manually update statistics on a table.

One is via the sp_updatestats system stored procedure. While the sp_updatestats procedure is an easier way to program the command, the UPDATE STATISTICS command is much more flexible and much more powerful.

By default, statistics are compiled based on a sampling of the data in the table.

The sampling amount varies depending on the size of the table, but it can be as little as just a few percent to as much as 100% of the table.

Also, if you have a very large table, it may take quite a while to reach the 10% metric -- longer than you want in some cases.