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SQL Server’s query optimizer uses distribution statistics to determine how it’s going to satisfy your SQL query. These statistics represent the distribution of the data within a column, or columns. The Query Optimizer uses them to estimate how many rows will be returned from a query plan.
Distribution statistics are created automatically when you create an index. If you have enabled the automatic creation of statistics (the default setting of the AUTO_CREATE_STATISTICS database setting ) you’ll also get statistics created any time a column is referenced in a query as part of a filtering clause or JOIN criteria.
Density is the easiest of the two to understand. Density is a ratio that shows just how many unique values there are within a given column, or set of columns. The formula is quite easy:
Density = 1 / Number of distinct values for column(s)
You can see the density for compound columns too. You just have to modify the query to get a distinct listing of your columns first:
The data distribution represents a statistical analysis of the kind of data that is in the first column available for statistics. That’s right, even with a compound index, you only get a single column of data for data distribution.
The rows represent the way the data is distributed within the column by showing a pieces of data describing that distribution:
|RANGE_HI_KEY||This is the top value of the step represented by this row within the histogram.|
|RANGE_ROWS||This number shows the number of rows within the step that are greater than the previous top value and the current top value, but not equal to either.|
|EQ_ROWS||This number shows the number of rows within the step that are greater than the previous top value and the current top value, but not equal to either.|
|DISTINCT_RANGE_ROWS||These are the distinct count of rows within a step. If all the rows are unique, then the RANGE_ROWS and the DISTINCT_RANGE_ROWS will be equal.|
|AVG_RANGE_ROWS||This represents the average number of rows equal to a key value within the step.
To see the current status of your statistics you use the DBCC statement SHOW_STATISTICS. The output is in three parts:
- Header: which contains meta-data about the set of statistics
- Density: Which shows the density values for the column or columns that define the set of statistics
- Histogram: The table that defines the histogram laid out above
You can pull individual pieces of this data out by modifying the DBCC statement.
Source of this article : https://www.simple-talk.com/sql/learn-sql-server/statistics-in-sql-server/