Django的F（）对象的性能问题

Question

有没有人注意到使用Django的F（）对象的性能问题？我正在运行Windows XP SP3并针对Django主干进行开发。我正在使用的模型片段和我正在构建的查询如下。当我有F（）对象时，每次调用QuerySet方法（例如filter，exclude，order_by，distinct等）大约需要2秒，但是当我注释掉F（）子句时，调用是第二。我有一个同事在他的Ubuntu机器上测试它，他没有遇到与F（）子句相同的性能问题。

其他人看到这种行为？

class Move (models.Model):
    state_meaning = models.CharField(
        max_length=16,
        db_index=True,
        blank=True,
        default=''
    )
    drop = models.ForeignKey(
        Org,
        db_index=True,
        null=False,
        default=1,
        related_name='as_move_drop'
    )

class Split(models.Model):
    state_meaning = models.CharField(
        max_length=16,
        db_index=True,
        blank=True,
        default=''
    )
    move = models.ForeignKey(
        Move,
        related_name='splits'
    )
    pickup = models.ForeignKey(
        Org,
        db_index=True,
        null=False,
        default=1,
        related_name='as_split_pickup'
    )
    pickup_date = models.DateField(
        null=True,
        default=None
    )
    drop = models.ForeignKey(
        Org,
        db_index=True,
        null=False,
        default=1,
        related_name='as_split_drop'
    )
    drop_date = models.DateField(
        null=True,
        default=None,
        db_index=True
    )

def get_splits(begin_date, end_date):
    qs = Split.objects \
        .filter(state_meaning__in=['INPROGRESS','FULFILLED'],
                drop=F('move__drop'), # <<< the line in question
                pickup_date__lte=end_date)

    elapsed = timer.clock() - start
    print 'qs1 took %.3f' % elapsed
    start = timer.clock()

    qs = qs.filter(Q(drop_date__gte=begin_date) |
                   Q(drop_date__isnull=True))

    elapsed = timer.clock() - start
    print 'qs2 took %.3f' % elapsed
    start = timer.clock()

    qs = qs.exclude(move__state_meaning='UNFULFILLED')

    elapsed = timer.clock() - start
    print 'qs3 took %.3f' % elapsed
    start = timer.clock()

    qs = qs.order_by('pickup_date', 'drop_date')

    elapsed = timer.clock() - start
    print 'qs7 took %.3f' % elapsed
    start = timer.clock()

    qs = qs.distinct()

    elapsed = timer.clock() - start
    print 'qs8 took %.3f' % elapsed