Constraints can and do help performance. Unique constraints may allow SQL to ignore DISTINCT operations or reduce group by statements to simpler forms or know absolutely for sure how many rows a particular operation will affect. Check constraints can allow SQL to ignore entire predicates (eg WHERE x > 0 when there's a constraint forcing x to be > 0) or if a predicate requires rows that violate constraints (where x IS NULL on a not null column). Similar with foreign keys.