Improvement: Improves the query performance for SQL Server 2016 by changing the use of histograms on UNIQUE columns


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.


This improvement is introduced in the following cumulative update for SQL Server:

Cumulative Update 4 for SQL Server 2016

About the cumulative updates for SQL Server


Learn about the terminology that Microsoft uses to describe software updates.

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