Answer
A unique index prevents multiple rows from having equal indexed key values under its null semantics. • It can enforce a uniqueness rule. • In PostgreSQL, UNIQUE and PRIMARY KEY constraints create unique indexes automatically. • Prefer a constraint when expressing a logical data rule.
💡 SQL Example
CREATE UNIQUE INDEX users_email_idx ON users (email);
Result
CREATE INDEX
⚡ Quick Revision
A unique index combines indexed lookup with uniqueness enforcement.