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根据（我的阅读）官方dox here：

https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated

评估时应缓存 Django QuerySet。但情况似乎并非如此。在下面的示例中，TrackingImport 是一个模型，其后面有一个非常大的表。（为简洁起见，稍微编辑了输出。）

recs = TrackingImport.objects.filter(...stuff...)

In [102]: time(recs[0])
Wall time: 1.84 s

In [103]: time(recs[0])
Wall time: 1.84 s

调用 len() 似乎像宣传的那样工作：

In [104]: len(recs)
Out[104]: 1823

In [105]: time(recs[0])
Wall time: 0.00 s

我不明白为什么取消引用数组没有缓存 QuerySet 结果。它必须评估它，对吗？那么我错过了什么？

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1 回答 1

7

你可以去看看源码（django.db.model.query），然后就清楚了，这里是django 1.3.4的query.py，

def __getitem__(self, k):
    """
    Retrieves an item or slice from the set of results.
    """
    if not isinstance(k, (slice, int, long)):
        raise TypeError
    assert ((not isinstance(k, slice) and (k >= 0))
            or (isinstance(k, slice) and (k.start is None or k.start >= 0)
                and (k.stop is None or k.stop >= 0))), \
            "Negative indexing is not supported."

    if self._result_cache is not None:
        if self._iter is not None:
            # The result cache has only been partially populated, so we may
            # need to fill it out a bit more.
            if isinstance(k, slice):
                if k.stop is not None:
                    # Some people insist on passing in strings here.
                    bound = int(k.stop)
                else:
                    bound = None
            else:
                bound = k + 1
            if len(self._result_cache) < bound:
                self._fill_cache(bound - len(self._result_cache))
        return self._result_cache[k]

    if isinstance(k, slice):
        qs = self._clone()
        if k.start is not None:
            start = int(k.start)
        else:
            start = None
        if k.stop is not None:
            stop = int(k.stop)
        else:
            stop = None
        qs.query.set_limits(start, stop)
        return k.step and list(qs)[::k.step] or qs
    try:
        qs = self._clone()
        qs.query.set_limits(k, k + 1)
        return list(qs)[0]
    except self.model.DoesNotExist, e:
        raise IndexError(e.args)

当您不遍历查询集时，_result_cache 为 None，然后当您调用 resc[0] 时，它只会跳到以下几行，

try:
   qs = self._clone()
   qs.query.set_limits(k, k + 1)
   return list(qs)[0]
except self.model.DoesNotExist, e:
   raise IndexError(e.args)

您会发现，在这种情况下，_result_cache 没有被设置。这就是为什么多个 resc[0] 的持续时间成本相同的原因。

调用 len(resc) 后，可以找到源代码，

def __len__(self):
    # Since __len__ is called quite frequently (for example, as part of
    # list(qs), we make some effort here to be as efficient as possible
    # whilst not messing up any existing iterators against the QuerySet.
    if self._result_cache is None:
        if self._iter:
            self._result_cache = list(self._iter)
        else:
            self._result_cache = list(self.iterator())
    elif self._iter:
        self._result_cache.extend(self._iter)
    return len(self._result_cache)

你可以看到 _result_cache 有值，然后你调用 recs[0]，它只会使用缓存，

 if self._result_cache is not None:
         ....
     return self._result_cache[k]

源代码永远不会说谎，因此当您在文档中找不到答案时，最好阅读源代码。

于 2012-12-31T01:08:34.997 回答