Redshift PK/FK constraints are planner hints, not enforced
When generating Redshift DDL, never rely on PK/FK/unique constraints for data integrity: enforce it in the ETL or with a MERGE dedup step instead. Still declare constraints you know are valid, since the planner uses them as hints for better plans. And do not copy a constraint-bearing DDL from Postgres to Redshift assuming the same enforcement semantics.
Context: AWS docs (Table constraints, Amazon Redshift): uniqueness, primary key, and foreign key constraints are informational only and are NOT enforced when you populate a table. An insert that violates the constraint still succeeds. The planner trusts declared keys as valid and uses them for join ordering and subquery decorrelation, so a false declaration can make queries return wrong results (e.g. SELECT DISTINCT returning duplicates). Only NOT NULL is actually enforced. Declare keys only when your ETL guarantees their integrity.Maintainer review
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