# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.contrib.postgres.aggregates.statistics

    from django.db.models import FloatField, IntegerField
    from django.db.models.aggregates import Aggregate

    __all__ = [
        'CovarPop', 'Corr', 'RegrAvgX', 'RegrAvgY', 'RegrCount', 'RegrIntercept',
        'RegrR2', 'RegrSlope', 'RegrSXX', 'RegrSXY', 'RegrSYY', 'StatAggregate',
    ]

    class StatAggregate(Aggregate):
        def __init__(self, y, x, output_field=FloatField()):
            if not x or not y:
                raise ValueError('Both y and x must be provided.')
            super(StatAggregate, self).__init__(y=y, x=x, output_field=output_field)
            self.x = x
            self.y = y
            self.source_expressions = self._parse_expressions(self.y, self.x)

        def get_source_expressions(self):
            return self.y, self.x

        def set_source_expressions(self, exprs):
            self.y, self.x = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            return super(Aggregate, self).resolve_expression(query, allow_joins, reuse, summarize)

    class Corr(StatAggregate):
        function = 'CORR'

    class CovarPop(StatAggregate):
        def __init__(self, y, x, sample=False):
            self.function = 'COVAR_SAMP' if sample else 'COVAR_POP'
            super(CovarPop, self).__init__(y, x)

    class RegrAvgX(StatAggregate):
        function = 'REGR_AVGX'

    class RegrAvgY(StatAggregate):
        function = 'REGR_AVGY'

    class RegrCount(StatAggregate):
        function = 'REGR_COUNT'

        def __init__(self, y, x):
            super(RegrCount, self).__init__(y=y, x=x, output_field=IntegerField())

        def convert_value(self, value, expression, connection, context):
            if value is None:
                return 0
            return int(value)

    class RegrIntercept(StatAggregate):
        function = 'REGR_INTERCEPT'

    class RegrR2(StatAggregate):
        function = 'REGR_R2'

    class RegrSlope(StatAggregate):
        function = 'REGR_SLOPE'

    class RegrSXX(StatAggregate):
        function = 'REGR_SXX'

    class RegrSXY(StatAggregate):
        function = 'REGR_SXY'

    class RegrSYY(StatAggregate):
        function = 'REGR_SYY'