In the current cardinality estimation model of Microsoft SQL Server 2016, uniqueness isn't explained when histograms are scaled. This may cause unique columns having a frequency that's greater than 1. The method for dealing with this in RTM is to ignore the histogram completely on UNIQUE columns. This may cause poor estimates when the distribution of the column is non-uniform.
After this update is installed, the histogram will be used properly if it has all step values within 10% of unique. This change will only be enabled if trace flag 4199 is enabled.
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