Data

Covering Index

An index containing every column a query needs, so the table is never read.

Why interviewers ask about it

Turns two reads into one and can be an order of magnitude faster on wide tables. The cost is a larger index and slower writes.

A composite index on (a, b, c) can be seeked by a, or a+b, or a+b+c - but never by b alone.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.