Django prefetch_related优化查询，但仍然很慢 [英] Django prefetch_related optimize query but still very slow

问题描述

在具有5个m2m字段的模型上， prefetch_related 遇到一些严重的性能问题，并且我还预取了一些嵌套的m2m字段.

I'm experiencing some severe performances issues with prefetch_related on a Model with 5 m2m fields and I'm pre-fetching also few nested m2m fields.

class TaskModelManager(models.Manager):
    def get_queryset(self):
        return super(TaskModelManager, self).get_queryset().exclude(internalStatus=2).prefetch_related("parent", "takes", "takes__flags", "assignedUser", "assignedUser__flags", "asset", "asset__flags", "status", "approvalWorkflow", "viewers", "requires", "linkedTasks", "activities")


class Task(models.Model):
    uuid = models.UUIDField(primary_key=True, default=genOptimUUID, editable=False)
    internalStatus = models.IntegerField(default=0)
    parent = models.ForeignKey("self", blank=True, null=True, related_name="childs")
    name = models.CharField(max_length=45)
    taskType = models.ForeignKey("TaskType", null=True)
    priority = models.IntegerField()
    startDate = models.DateTimeField()
    endDate = models.DateTimeField()
    status = models.ForeignKey("ProgressionStatus")
    assignedUser = models.ForeignKey("Asset", related_name="tasksAssigned")
    asset = models.ForeignKey("Asset", related_name="tasksSubject")
    viewers = models.ManyToManyField("Asset", blank=True, related_name="followedTasks")
    step = models.ForeignKey("Step", blank=True, null=True, related_name="tasks")
    approvalWorkflow = models.ForeignKey("ApprovalWorkflow")
    linkedTasks = models.ManyToManyField("self", symmetrical=False, blank=True, related_name="linkedTo")
    requires = models.ManyToManyField("self", symmetrical=False, blank=True, related_name="depends")

    objects = TaskModelManager()

查询次数很好，数据库查询时间也很好，例如，如果我查询模型的700个对象，我有35个查询，平均查询时间为100〜200ms，但总请求时间约为8秒

The number of query is fine and the database query time is fine too, for exemple if I query 700 objects of my model i have 35 query and the average query time is 100~200ms but the total request time is approximately 8 seconds.

丝绸时间

我已经进行了一些分析，并指出花费了80％以上的时间是在 prefetch_related_objects 调用上.

I've run some profiling and it pointed out that more than 80% of the time spent was on the prefetch_related_objects call.

分析

我正在使用 Django == 1.8.5 和 djangorestframework == 3.4.6

我愿意采取任何方式对此进行优化.预先感谢您的帮助.

I'm open to any way to optimize this. Thanks in advance for your help.

使用 select_related

Edit with select_related:

我已经尝试过阿拉斯达尔提出的改进措施

I've tried the improvement proposed by Alasdair

class TaskModelManager(models.Manager):
    def get_queryset(self):
        return super(TaskModelManager, self).get_queryset().exclude(internalStatus=2).select_related("parent", "status", "approvalWorkflow", "step").prefetch_related("takes", "takes__flags", "assignedUser", "assignedUser__flags", "asset", "asset__flags", "viewers", "requires", "linkedTasks", "activities")

对于32个查询和150ms查询时间的请求，新结果仍然是8秒.

The new result is still 8 seconds for the request with 32 queries and 150ms of query time.

似乎四年前在Django问题跟踪器上打开了一个票证，并且该票证仍在打开:https://code.djangoproject.com/ticket/20577

It seems that a ticket was opened on Django issue tracker 4 years ago and is still open : https: //code.djangoproject.com/ticket/20577

推荐答案

我遇到了同样的问题.

在您链接的问题之后，我发现您可以改善 prefetch_related Prefetch 对象和 to_attr 自变量来实现code>性能.

Following the issue you linked i found that you can improve the prefetch_related performance using Prefetch object and to_attr argument.

从提交中引入，该对象引入了 Prefetch 对象:

From the commit that introduces the Prefetch object:

当Prefetch实例指定to_attr参数时，结果为存储在列表中而不是QuerySet中.这很幸运明显更快的结果.性能改进是由于我们节省了创建查询集的昂贵开销实例.

When a Prefetch instance specifies a to_attr argument, the result is stored in a list rather than a QuerySet. This has the fortunate consequence of being significantly faster. The preformance improvement is due to the fact that we save the costly creation of a QuerySet instance.

因此，只需调用以下命令，我就可以将我的代码从7秒显着提高到0.88秒:

So i significantly improved my code (from about 7 seconds to 0.88 seconds) simply by calling:

Foo.objects.filter(...).prefetch_related(Prefetch('bars', to_attr='bars_list'))

代替

Foo.objects.filter(...).prefetch_related('bars')

这篇关于Django prefetch_related优化查询，但仍然很慢的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！