Sql updating statistics
To view this histogram, go to the details tab of the statistic’s properties or use the command DBCC SHOW_STATISTICS.
The screenshot below shows the histogram of an index statistic; the RANGE_HI_KEY is the upper limit of the step, the RANGE_HI_KEY of the previous step 1 is the lower limit, and the RANGE_ROWS is the count of rows between the limits.
Updating statistics for a large database can easily take hours, even days, to complete.
When Auto Create Statistics is enabled, stats can be created even for a one time query.
Column statistics: Created manually by the DBA using the ‘CREATE STATISTICS’ command, or automatically if the “Auto Create Statistics” option is set to “True”.
Column statistics can be created, modified and dropped at will.
When the data in the database changes the statistics become stale and outdated.
When examining a query execution plan, a large discrepancy between the Actual Number of Rows and the Estimated Number of Rows is an indication of outdated stats.
Scheduling the statistics maintenance during off hours will help reduce the need to update statistics during peak times.Statistics are SQL Server objects which contain metrics on the data count and distribution within a column or columns used by the optimizer to help it make that choice. Index statistics: Created automatically when an index (both clustered and non-clustered) is created.These will have the same name as the index and will exist as long as the index exists.There are multiple paths a database can use to answer a query, some of them being faster and more efficient than others.It is the job of the query optimizer to evaluate and choose the best path, or execution plan, for a given query.
Using the available indexes may not always be the most efficient plan.