Amazon Redshift suggests to define primary key or foreign key constraint wherever applicable. But they are information only. Redshift does not enforce these constraints. In other words, a column with primary key accepts duplicate values as well as a foreign key column also allows such a value that does not exists in the referenced table. So why does Redshift consider defining primary or foreign key as their best practice list? Because query optimizer uses them to choose the most suitable execution plan. But you need to be very careful while defining these constraint in your table. Let us see why with some real-life example.
