[qaq]

in terms of ACID "Consistency ensures that a transaction can only bring the database from one consistent state to another, preserving database invariants: any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof. This prevents database corruption by an illegal transaction. Referential integrity guarantees the primary key–foreign key relationship." So no it's not free

[jeremyjh]

That is an entirely different meaning of consistency. GP was talking about CAP theory, as they acknowledged in a follow-up comment.