Does validates :uniqueness get called even if a field has not changed?

validations.rb

# Does validates :uniqueness get called even if a field has not changed?
# Isn't this a performance issue?

validates :name, :schedule_id, :uniqueness => true

# My experience that it does. So isn't it almost always necessary to make sure a change has taken
# place before running the validation? As every field being checked for uniqueness requires a 
# database hit.

# This would be better:

validates :name, :schedule_id, :uniqueness => true, :if => "name_changed? || schedule_id_changed?"

# And this much better, if a bit more verbose:

validates :name, :uniqueness => true, :if => :name_changed?
validates :schedule_id, :uniqueness => true, :if => schedule_id_changed?