Answer
An index is a database structure that can speed access to table rows. • It stores indexed key values with references that help locate rows. • The optimizer chooses whether an index is useful for a query. • Indexes consume storage and add work to data changes.
💡 SQL Example
CREATE INDEX employees_department_idx ON employees (department_id);
Result
CREATE INDEX
⚡ Quick Revision
An index trades storage and write cost for potentially faster reads.