What is a covering index?

A covering index is an index that contains all the columns needed by a query, meaning the

query can be answered entirely by the index without accessing the actual table data. This

can improve query performance by reducing disk I/O.

Example:

If a query selects id, name, and salary from the employees table and there is an index

on (id, name, salary), the index itself is sufficient to satisfy the query, and the

database doesn't need to access the table at all.