Chapter 46. Using correlation to improve query performance
SQL Server doesn’t keep statistics on the correlation between nonclustered indexes and the clustered index (with the exception of correlation information between datetime columns, if the DATE_CORRELATION_OPTIMIZATION setting is turned on). Instead, the optimizer assumes it has a low correlation; it assumes that a range of nonclustered index values is scattered all over the clustered index.
This assumption affects the optimizer’s decision whether or not to use the nonclustered index. If there is a high correlation, the optimizer will overestimate the cost of using the nonclustered index, which can cause it to disqualify the index from the query plan evaluation, resulting ...