SQL Interview: 49 Index impacts on update operations

This is a post in the SQL Interview series. These aren’t trick or gotcha questions, they’re just questions designed to scope out a candidate’s knowledge around SQL Server and Azure SQL Database.

Section: Administrator Level: Medium

Question:

You are considering adding an index to a table.

You have heard that adding indexes slows down UPDATE commands.

Does adding an index always slow down updates? If so, why? If not, why not?

Answer:

When you add an index to a table, you have added something that might need to be updated as well as the table. This is why people are concerned about the impact.

However, it does not always slow down updates.

• The index might be based on columns that are unaffected by the UPDATE
• The index might have a positive impact on helping to find the rows that need to be updated.

2025-05-20