indexing - SQL Server unique vs not unique index INSERT performance

Question

I would like to create a covering index to improve query performance. I do know that this one more index will impact INSERT performance.

The table has only INSERT operations no UPDATE or DELETE. The data in covering index will unique because the index keys contain the table's PK, so I need no further constraining uniqueness, my only goal to improve query performance.

Question

Which type of index will the optimal (means degrade less) the INSERT performance, unique or not unique?

question from:https://stackoverflow.com/questions/65871602/sql-server-unique-vs-not-unique-index-insert-performance

Answer 1

Which type of index will the optimal (means degrade less) the INSERT performance, unique or not unique?

The difference if any is negligible.

On the other hand, what could have some impact is the index key length. Smaller is better. You could remove the primary key from the index key and make the index non unique. It will remain being a covering index because all non clustered indexes have the clustered index key on leaf page nodes, assuming your primary key is clustered.