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Say I have an ID row (int) in a database set as the primary key. If I query off the ID often do I also need to index it? Or does it being a primary key mean it’s already indexed?
Reason I ask is because in MS SQL Server I can create an index on this ID, which as I stated is my primary key.
Edit: an additional question – will it do any harm to additionally index the primary key?
You are right, it’s confusing that SQL Server allows you to create duplicate indexes on the same field(s). But the fact that you can create another doesn’t indicate that the PK index doesn’t also already exist.
The additional index does no good, but the only harm (very small) is the additional file size and row-creation overhead.