Redshift is a columnar database better suited for analytics, and thus a more appropriate platform for a data warehouse.
In PostgreSQL a single database connection cannot utilize more than one CPU, while Redshift is architected for parallel processing across multiple nodes.
Redshift doesn’t enforce primary key, foreign key, or uniqueness constraints, though Amazon says “primary keys and foreign keys are used as planning hints and they should be declared if your ETL process or some other process in your application enforces their integrity.”