django的orm操作优化

django的orm操作优化

models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=16)
    age = models.IntegerField()

class Book(models.Model):
    authors = models.ManyToManyField('Author')
    pubs = models.ForeignKey("Publish",on_delete=models.CASCADE)
    title = models.CharField(max_length=32)
    pubtime = models.DateField()

class Publish(models.Model):
    name = models.CharField(max_length=32)

优化一：尽量不查对象，能用values就是用values

直接使用对象查询的结果是5条sql语句

def youhua(request):
    # 使用对象查
    obj_list = models.Book.objects.all()
    for obj in obj_list:
        print(obj.title,obj.pubs.name)
        
    return render(request,"youhua.html")

使用values查询只执行了1条sql，会自动进行连表查询

def youhua(request):
    # 使用values
    obj_list = models.Book.objects.values('title', 'pubs__name')
    for obj in obj_list:
        print(obj['title'], obj['pubs__name'])

    return render(request,"youhua.html")

优化二：select_related('classes')

使用select_related('classes')

适用于：多对一 ，一对一查询添加select_related()方法，括号中是外键字段 。会进行连表查询

def youhua(request):
    # 使用对象查
    obj_list = models.Book.objects.all().select_related('pubs')
    for obj in obj_list:
        print(obj.title, obj.pubs.name)

    return render(request,"youhua.html")

优化三：prefetch_related()

使用prefetch_related()，多对一，

def youhua(request):

    obj_list = models.Book.objects.all().prefetch_related("pubs")
    for obj in obj_list:
        print(obj.title,obj.pubs.name)

    return render(request,"youhua.html")

使用prefetch_related()，多对多，

def youhua(request):

    obj_list = models.Book.objects.all()
    for obj in obj_list:
        print(obj.title,obj.authors.all())

    return render(request,"youhua.html")

def youhua(request):

    obj_list = models.Book.objects.all().prefetch_related("authors")
    for obj in obj_list:
        print(obj.title,obj.authors.all())

    return render(request,"youhua.html")

优化四：only()指定查询字段

直接查询的情况，会将所有字段都查询出来

def youhua(request):

    obj_list = models.Book.objects.all()
    for obj in obj_list:
        print(obj.title)

    return render(request,"youhua.html")

查询时指定某些字段查询，使用only指定字段只会查我们需要的那个字段。

def youhua(request):

    obj_list = models.Book.objects.all().only('title')
    for obj in obj_list:
        print(obj.title)

    return render(request,"youhua.html")

优化五：defer() 查询时指定排除某些字段和only相反

如果我们需要的字段较多，不需要的较少，可以使用defer排除

def youhua(request):

    obj_list = models.Book.objects.all().defer('title')
    for obj in obj_list:
        print(obj.pubtime)

    return render(request,"youhua.html")

注：如果排除的字段，还要查会增加查询负担，当然查询指定字段之外的字段也会增加查询负担。