A MERGE statement can fail, and incorrectly report a unique key violation when: The target table uses a unique filtered index; and No key column of the filtered index is updated; and A column from the filtering condition is updated; and Transient key violations are possible Example Tables Say we have ...

Estimated row counts on key or RID lookups where a filtering predicate is applied can be wrong in SSMS execution plans.  This error does not affect the optimizer’s ultimate plan selection, but it does look odd.  There are other cases where estimated row counts are inconsistent (for defensible reasons) but the behaviour shown in this post ...