Similar to the way you use filtered indexes, SQL Server 2008 also lets you create filtered statistics. Like filtered indexes, filtered statistics are also created over a subset of rows in the table based on a specified filter predicate. Creating a filtered index on a column autocreates the corresponding filtered statistics. In addition, filtered statistics can be created explicitly by including the
WHERE clause with the
CREATE STATISTICS statement.
Filtered statistics can be used to avoid a common issue with statistics where the cardinality estimation is skewed due to a large number of
NULL or duplicate values, or due to a data correlation between columns. For example, let’s consider the
titles table in ...