IGNORE_DUP_KEY slower on clustered indexes
SQL Performance
APRIL 5, 2019
The IGNORE_DUP_KEY option for unique indexes specifies how SQL Server responds to an attempt to INSERT duplicate values: It only applies to tables (not views) and only to inserts. Any insert portion of a MERGE statement ignores any IGNORE_DUP_KEY index setting. When IGNORE_DUP_KEY is OFF , the first duplicate encountered results in an error , and none of the new rows are inserted.
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