Django之ORMselect_related和prefetch_related

# 普通查询
q = models.UserInfo.objects.all()
for row in q:
    print(row.name, row.ut.title)  # 每次循环时都会去连表查询


# select_related: 查询主动做连表     适合表数据不是特别大
q = models.UserInfo.objects.all().select_related('ut') # ut字段是UserInfo表中ForiginKey外键关联字段
# 相当于  select * from userinfo inner join usertype on ...
for row in q:
    print(row.name, row.ut.title) # 加了select_related()就不会每次去联表查询了
    
     
# prefetch_related: 不做连表， 做多次单表查询   适合表数据非常大
q = models.UserInfo.objects.all().prefetch_related('ut')
# select * from userinfo
# Django内部： ut_id = [2, 4]     把所有的ut_id整合到一个集合做去重
# select * from usertype where id in [2, 4]
for row in q:
    print(row.id, row.ut.title)