'Defines time granularity database-specific expressions.
The idea here is to make it easy for users to change the time grain
form a datetime (maybe the source grain is arbitrary timestamps, daily
or 5 minutes increments) to another, "truncated" datetime. Since
each database has slightly different but similar datetime functions,
this allows a mapping between database engines and actual functions.'

def grains(self):

    return self.db_engine_spec.time_grains
