I have just published a four-part series for SQLPerformance.com on the Halloween Problem. Some of you will never have heard of this issue, and those that have might associate it only with T-SQL UPDATE queries. In fact, the Halloween problem affects execution plans for INSERT, UPDATE, DELETE and MERGE statements. This is a topic I have been ...

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 ...