When can an index hurt performance?

When indexes hurt

For When can an index hurt performance, interviewers listen for a crisp definition, how the pieces interact in a real system, and one trade-off or failure mode you have seen or would guard against.

• Tiny tables—sequential scan cheaper.
• Heavy write tables—each insert updates indexes.
• Low-cardinality columns alone (boolean) may not help.
• Wrong index prevents better plan—analyze queries.