Constraints

Tables that have been upgraded to crrs may not have:

1. Checked Foreign key Constraints. Foreign keys and joins are allowed but the constraint cannot be checked.
2. Unique constraints that are not the primary key
3. Check constraints that depend on columns other than the defined column

While there are techniques that allow preserving checked foreign keys, even under conditions created by eventual consistency, they all have nuanced tradeoffs. Throwing out these constraints for crrs is something that is clear and keeps the model easy to undestand.