PostgreSQL for Structured Scraped Data

Why does the lesson model the products table with a composite UNIQUE on (source, source_id) rather than a global UNIQUE on source_id?

• APostgres can't enforce uniqueness on a single column
• BTwo different sources can legitimately reuse the same source_id; the composite key prevents collisions across sources
• CComposite keys are always faster
• DSingle-column unique constraints require a separate table