本文将通过简单的示例介绍为什么要使用prefetch_related
。
准备工作
创建模型
python
from django.db import models
class Product(models.Model):
product_name = models.CharField(max_length=255)
class Component(models.Model):
product = models.ForeignKey(Product, on_delete=models.CASCADE, related_name='components')
code = models.CharField(max_length=255)
class Meta:
ordering = ['product']
创建数据
python
>>> from api.models import Product
>>> p1 = Product.objects.create(product_name="产品1")
>>> p2 = Product.objects.create(product_name="产品2")
>>> from api.models import Component
>>> pc1 = Component.objects.create(product=p1, code="LS")
>>> pc2 = Component.objects.create(product=p1, code="LM")
>>> pc3 = Component.objects.create(product=p2, code="XYB")
>>> pc4 = Component.objects.create(product=p2, code="LXC")
在settings.py
文件中添加配置输出运行时执行的sql
python
LOGGING = {
'version': 1,
'disable_existing_loggers': False,
'handlers': {
'console': {
'class': 'logging.StreamHandler',
},
},
'loggers': {
'django.db.backends': {
'handlers': ['console'],
'level': 'DEBUG',
},
},
}
不使用prefetch_related
python
from api.models import Product
ps = Product.objects.all()
# SELECT "api_product"."id", "api_product"."product_name" FROM "api_product"; args=(); alias=default
for p in ps:
print(p.components.all())
# SELECT "api_component"."id", "api_component"."product_id", "api_component"."code" FROM "api_component" WHERE "api_component"."product_id" = 1 ORDER BY "api_component"."product_id" ASC LIMIT 21; args=(1,);
# SELECT "api_component"."id", "api_component"."product_id", "api_component"."code" FROM "api_component" WHERE "api_component"."product_id" = 2 ORDER BY "api_component"."product_id" ASC LIMIT 21; args=(2,);
不使用prefetch_related
我们获取所有"产品"执行了一次sql查询,然而当我们遍历想要获取所有产品的组件时,对每个产品都执行了一次sql查询。
我们的示例中只产品的数量只有两个,当产品的数量非常多,成百上千时,对每个产品你都要到数据库执行一次查询,这将导致严重的性能问题,也就是所谓的n+1问题。
使用prefetch_related
python
from api.models import Product
ps = Product.objects.prefetch_related("components")
# SELECT "api_product"."id", "api_product"."product_name" FROM "api_product"; args=(); alias=default
# SELECT "api_component"."id", "api_component"."product_id", "api_component"."code" FROM "api_component" WHERE "api_component"."product_id" IN (1, 2) ORDER BY "api_component"."product_id" ASC; args=(1, 2); alias=default
for p in ps:
print(p.components.all())
可以清晰的看到总共执行了两次sql查询,在获取全部"产品"的时候通过一次额外的查询将所有产品的组件信息缓存起来,当我们需要的时候是从缓存中获取,不会执行任何的数据库查询
select_related
select_related
只能用于一对一关系和外键关系,它是利用Join只进行一次SQL查询,但是它的局限也很明显,不能用于多对多,多对一等
python
from api.models import Component
c = Component.objects.select_related("product").get(pk=1)
# SELECT "api_component"."id", "api_component"."product_id", "api_component"."code", "api_product"."id", "api_product"."product_name" FROM "api_component" INNER JOIN "api_product" ON ("api_component"."product_id" = "api_product"."id") WHERE "api_component"."id" = 1 LIMIT 21; args=(1,); alias=default