查询优化之select_related与prefetch_rel

表数据如下（这段可以不看）：

from django.db import models
from django.contrib.auth.models import AbstractUser

class UserInfo(AbstractUser):  # settings:   AUTH_USER_MODEL = "blog.UserInfo"
    """
    用户信息
    """
    nid = models.BigAutoField(primary_key=True)
    nickname = models.CharField(verbose_name='昵称', max_length=32,null=True,blank=True)
    telephone = models.CharField(max_length=11, blank=True, null=True, unique=True, verbose_name='手机号码')
    avatar = models.FileField(verbose_name='头像', upload_to='avatar', default="/avatar/default.jpg")
    create_time = models.DateTimeField(verbose_name='创建时间', auto_now_add=True)

    def __str__(self):
        return self.username


class Blog(models.Model):
    """
    站点信息
    """
    nid = models.BigAutoField(primary_key=True)
    title = models.CharField(verbose_name='个人博客标题', max_length=64)
    site = models.CharField(verbose_name='个人博客后缀', max_length=32, unique=True)
    theme = models.CharField(verbose_name='博客主题', max_length=32)

    user = models.OneToOneField(to='UserInfo', to_field='nid')

    def __str__(self):
        return self.title


class Category(models.Model):
    """
    博主个人文章分类表
    """
    nid = models.AutoField(primary_key=True)
    title = models.CharField(verbose_name='分类标题', max_length=32)
    blog = models.ForeignKey(verbose_name='所属博客', to='Blog', to_field='nid')

    def __str__(self):
        return self.title


class Article(models.Model):
    """
    文章表
    """
    nid = models.BigAutoField(primary_key=True)
    title = models.CharField(max_length=50, verbose_name='文章标题')
    desc = models.CharField(max_length=255, verbose_name='文章描述')
    read_count = models.IntegerField(default=0)
    comment_count = models.IntegerField(default=0)
    up_count = models.IntegerField(default=0)
    down_count = models.IntegerField(default=0)
    create_time = models.DateTimeField(verbose_name='创建时间', auto_now_add=True)

    category = models.ForeignKey(verbose_name='文章类型', to='Category', to_field='nid', null=True)
    blog = models.ForeignKey(verbose_name='所属博客', to='Blog', to_field='nid')
    tags = models.ManyToManyField(
        to="Tag",
        through='Article2Tag',
        through_fields=('article', 'tag'),
    )

    siteArticleCategory = models.ForeignKey(to='SiteArticleCategory', verbose_name='文章所属网站分类',null=True,blank=True)

    # type_choices = [
    #     (1, "编程语言"),
    #     (2, "软件设计"),
    #     (3, "前端"),
    #     (4, "操作系统"),
    #     (5, "数据库"),
    # ]
    # article_type_id = models.IntegerField(choices=type_choices, default=None)

    def __str__(self):
        return self.title

class SiteCategory(models.Model):
    """
    网站分类
    """
    title = models.CharField(max_length=50, verbose_name='分类名称')

    def __str__(self):
        return self.title

class SiteArticleCategory(models.Model):
    """
    网站文章分类
    """
    title = models.CharField(max_length=50, verbose_name='二级分类名称')
    siteCategory = models.ForeignKey(to='SiteCategory', verbose_name='所属网站分类')

    def __str__(self):
        return self.title


class ArticleDetail(models.Model):
    """
    文章详细表
    """
    nid = models.AutoField(primary_key=True)
    content = models.TextField(verbose_name='文章内容', )
    article = models.OneToOneField(verbose_name='所属文章', to='Article', to_field='nid')

    def __str__(self):
        return self.content


class Comment(models.Model):
    """
    评论表
    """
    nid = models.BigAutoField(primary_key=True)
    article = models.ForeignKey(verbose_name='评论文章', to='Article', to_field='nid')
    content = models.CharField(verbose_name='评论内容', max_length=255)
    create_time = models.DateTimeField(verbose_name='创建时间', auto_now_add=True)

    parent_comment = models.ForeignKey('self', blank=True, null=True, verbose_name='父级评论')
    user = models.ForeignKey(verbose_name='评论者', to='UserInfo', to_field='nid')

    up_count = models.IntegerField(default=0)

    def __str__(self):
        return self.content


class ArticleUpDown(models.Model):
    """
    文章点赞表
    """
    nid = models.AutoField(primary_key=True)
    user = models.ForeignKey('UserInfo', null=True)
    article = models.ForeignKey("Article", null=True)
    is_up = models.BooleanField(verbose_name='是否赞',default=1)

    def __str__(self):
        return self.user.nickname+self.article.title

    class Meta:
        unique_together = [
            ('article', 'user','is_up'),
        ]



class CommentUpDown(models.Model):
    """
    评论点赞表
    """
    nid = models.AutoField(primary_key=True)
    user = models.ForeignKey('UserInfo', null=True)
    comment = models.ForeignKey("Comment", null=True)
    is_up = models.BooleanField(verbose_name='是否赞',default=1)

    def __str__(self):
        return self.nid


class Tag(models.Model):
    """
    博主个人文章标签表
    """
    nid = models.AutoField(primary_key=True)
    title = models.CharField(verbose_name='标签名称', max_length=32)
    blog = models.ForeignKey(verbose_name='所属博客', to='Blog', to_field='nid')

    def __str__(self):
        return self.title


class Article2Tag(models.Model):
    nid = models.AutoField(primary_key=True)
    article = models.ForeignKey(verbose_name='文章', to="Article", to_field='nid')
    tag = models.ForeignKey(verbose_name='标签', to="Tag", to_field='nid')

    class Meta:
        unique_together = [
            ('article', 'tag'),
        ]

select_related

简单使用

对于一对一字段（OneToOneField）和外键字段（ForeignKey），可以使用select_related 来对QuerySet进行优化。
select_related 返回一个QuerySet，当执行它的查询时它沿着外键关系查询关联的对象的数据。它会生成一个复杂的查询并引起性能的损耗，但是在以后使用外键关系时将不需要数据库查询。
简单说，在对QuerySet使用select_related()函数后，Django会获取相应外键对应的对象，从而在之后需要的时候不必再查询数据库了。

下面的例子解释了普通查询和select_related() 查询的区别。
查询主键为1的文章的分类名称,下面是一个标准的查询：

# Hits the database.
article = models.Article.objects.get(pk=1)

# Hits the database again to get the related Blog object.
print(article.category.title)

sql语句如下：

SELECT "blog_article"."nid",
   "blog_article"."title",
   "blog_article"."desc",
   "blog_article"."read_count", 
   "blog_article"."comment_count",
   "blog_article"."up_count",
   "blog_article"."down_count",
   "blog_article"."create_time", 
   "blog_article"."category_id",
   "blog_article"."blog_id",
   "blog_article"."siteArticleCategory_id" 
   FROM "blog_article" 
   WHERE "blog_article"."nid" = 1; args=(1,)


SELECT "blog_category"."nid",
   "blog_category"."title", 
   "blog_category"."blog_id" 
   FROM "blog_category" 
   WHERE "blog_category"."nid" = 2; args=(2,)

如果我们使用select_related()函数：

# Hits the database.
article = models.Article.objects.select_related('category').get(pk=1)

#Doesn't hit the database, because article_obj.category has been prepopulated in the previous query.
print(article.category.title)

SELECT "blog_article"."nid", 
  "blog_article"."title", 
  "blog_article"."desc",
  "blog_article"."read_count", 
  "blog_article"."comment_count", 
  "blog_article"."up_count",
  "blog_article"."down_count", 
  "blog_article"."create_time", 
  "blog_article"."category_id", 
  "blog_article"."blog_id",   
  "blog_article"."siteArticleCategory_id", 
  "blog_category"."nid", 
  "blog_category"."title", 
  "blog_category"."blog_id" 
  FROM "blog_article" 
  LEFT OUTER JOIN "blog_category" ON ("blog_article"."category_id" = "blog_category"."nid") 
  WHERE "blog_article"."nid" = 1; args=(1,)

多外键查询

这是针对category的外键查询，如果是另外一个外键呢？让我们一起看下：

article=models.Article.objects.select_related("category").get(nid=1)
print(article.articledetail)

SELECT "blog_article"."nid", 
  "blog_article"."title", 
  "blog_article"."desc", 
  "blog_article"."read_count", 
  "blog_article"."comment_count", 
  "blog_article"."up_count", 
  "blog_article"."down_count", 
  "blog_article"."create_time", 
  "blog_article"."category_id", 
  "blog_article"."blog_id", 
  "blog_article"."siteArticleCategory_id", 
  "blog_category"."nid", 
  "blog_category"."title", 
  "blog_category"."blog_id" 
  FROM "blog_article" 
  LEFT OUTER JOIN "blog_category" ON ("blog_article"."category_id" = "blog_category"."nid") 
  WHERE "blog_article"."nid" = 1; args=(1,)


SELECT "blog_articledetail"."nid", 
  "blog_articledetail"."content", 
  "blog_articledetail"."article_id" 
  FROM "blog_articledetail" 
  WHERE "blog_articledetail"."article_id" = 1; args=(1,)

观察logging结果，发现依然需要查询两次，所以需要改为：

article=models.Article.objects.select_related("category","articledetail").get(nid=1)
# 可变长参数的方式
print(article.articledetail)

或者

article = models.Article.objects.select_related("category").select_related("articledetail").get(nid=1)
# django 1.7之后支持链式操作
print(article.articledetail)

它们的sql语句一样，都是：

SELECT
 
    "blog_article"."nid",
    "blog_article"."title",
    ......
 
    "blog_category"."nid",
    "blog_category"."title",
    "blog_category"."blog_id",
 
    "blog_articledetail"."nid",
    "blog_articledetail"."content",
    "blog_articledetail"."article_id"
 
   FROM "blog_article"
   LEFT OUTER JOIN "blog_category" ON ("blog_article"."category_id" = "blog_category"."nid")
   LEFT OUTER JOIN "blog_articledetail" ON ("blog_article"."nid" = "blog_articledetail"."article_id")
   WHERE "blog_article"."nid" = 1; args=(1,)

深层查询

# 查询id=1的文章的用户姓名
article = models.Article.objects.select_related("blog").get(nid=1)
print(article.blog.user.username)

依然需要查询两次：

SELECT
    "blog_article"."nid",
    "blog_article"."title",
    ......
 
     "blog_blog"."nid",
     "blog_blog"."title",
 
   FROM "blog_article" INNER JOIN "blog_blog" ON ("blog_article"."blog_id" = "blog_blog"."nid")
   WHERE "blog_article"."nid" = 1;


SELECT
    "blog_userinfo"."password",
    "blog_userinfo"."last_login",
    ......
 
  FROM "blog_userinfo"
  WHERE "blog_userinfo"."nid" = 1;

这是因为第一次查询没有query到userInfo表，所以，修改如下：

article=models.Article.objects.select_related("blog__user").get(nid=1)
print(article.blog.user.username)

SELECT
  "blog_article"."nid", "blog_article"."title",
  ......
  "blog_blog"."nid", "blog_blog"."title",
  ......
  "blog_userinfo"."password", "blog_userinfo"."last_login",
  ......
  FROM "blog_article"
  INNER JOIN "blog_blog" ON ("blog_article"."blog_id" = "blog_blog"."nid")
  INNER JOIN "blog_userinfo" ON ("blog_blog"."user_id" = "blog_userinfo"."nid")
  WHERE "blog_article"."nid" = 1;

总结

• select_related主要针一对一和多对一关系进行优化。
• select_related使用SQL的JOIN语句进行优化，通过减少SQL查询的次数来进行优化、提高性能。
• 可以通过可变长参数指定需要select_related的字段名。也可以通过使用双下划线“__”连接字段名来实现指定的递归查询。
• 没有指定的字段不会缓存，没有指定的深度不会缓存，如果要访问的话Django会再次进行SQL查询。
• 也可以通过depth参数指定递归的深度，Django会自动缓存指定深度内所有的字段。如果要访问指定深度外的字段，Django会再次进行SQL查询。
• 也接受无参数的调用，Django会尽可能深的递归查询所有的字段。但注意有Django递归的限制和性能的浪费。
• Django >= 1.7，链式调用的select_related相当于使用可变长参数。Django < 1.7，链式调用会导致前边的select_related失效，只保留最后一个。

prefetch_related

对于多对多字段（ManyToManyField）和一对多字段，可以使用prefetch_related()来进行优化。
prefetch_related()和select_related()的设计目的很相似，都是为了减少SQL查询的数量，但是实现的方式不一样。后者是通过JOIN语句，在SQL查询内解决问题。但是对于多对多关系，使用SQL语句解决就显得有些不太明智，因为JOIN得到的表将会很长，会导致SQL语句运行时间的增加和内存占用的增加。若有n个对象，每个对象的多对多字段对应Mi条，就会生成Σ(n)Mi 行的结果表。
prefetch_related()的解决方法是，分别查询每个表，然后用Python处理他们之间的关系。

# 查询文章关联的所有标签
article_obj = models.Article.objects.filter(nid__in=(1,2))
for i in article_obj:
    print(i.tags.all())  # 2篇文章: hits database 3

SELECT "blog_article"."nid", 
  "blog_article"."title", 
  "blog_article"."desc", 
  "blog_article"."read_count", 
  "blog_article"."comment_count", 
  "blog_article"."up_count", 
  "blog_article"."down_count", 
  "blog_article"."create_time", 
  "blog_article"."category_id", 
  "blog_article"."blog_id", 
  "blog_article"."siteArticleCategory_id" 
  FROM "blog_article" 
  WHERE "blog_article"."nid" IN (1, 2); args=(1, 2)


SELECT "blog_tag"."nid", 
  "blog_tag"."title", 
  "blog_tag"."blog_id" 
  FROM "blog_tag" 
  INNER JOIN "blog_article2tag" ON ("blog_tag"."nid" = "blog_article2tag"."tag_id")     
  WHERE "blog_article2tag"."article_id" = 1 LIMIT 21;args=(1,)


SELECT "blog_tag"."nid", 
  "blog_tag"."title", 
  "blog_tag"."blog_id" 
  FROM "blog_tag" 
  INNER JOIN "blog_article2tag" ON ("blog_tag"."nid" = "blog_article2tag"."tag_id") 
  WHERE "blog_article2tag"."article_id" = 2 LIMIT 21;args=(2,)

改为prefetch_related：

# 查询文章关联的所有标签
article_obj = models.Article.objects.prefetch_related("tags").filter(nid__in=(1,2,3,4))
for i in article_obj:
    print(i.tags.all())  # 4篇文章: hits database 2

SELECT "blog_article"."nid",
  "blog_article"."title",
  ......
  FROM "blog_article"
  WHERE "blog_article"."nid" IN (1,2,3,4);
 
 
 
SELECT
  ("blog_article2tag"."article_id") AS "_prefetch_related_val_article_id",
  "blog_tag"."nid",
  "blog_tag"."title",
  "blog_tag"."blog_id"
  FROM "blog_tag"
  INNER JOIN "blog_article2tag" ON ("blog_tag"."nid" = "blog_article2tag"."tag_id")
  WHERE "blog_article2tag"."article_id" IN (1,2,3,4);