django_orm查询性能优化方法

models.Tb1.objects.create(c1='xx', c2='oo') 增加一条数据，可以接受字典类型数据 **kwargs
 
obj = models.Tb1(c1='xx', c2='oo')
obj.save()

models.Tb1.objects.get(id=123)     # 获取单条数据，不存在则报错（不建议）
models.Tb1.objects.all()        # 获取全部
models.Tb1.objects.filter(name='seven') # 获取指定条件的数据
models.Tb1.objects.exclude(name='seven') # 获取指定条件的数据

models.Tb1.objects.filter(name='seven').delete() # 删除指定条件的数据

models.Tb1.objects.filter(name='seven').update(gender='0') # 将指定条件的数据更新，均支持 **kwargs
obj = models.Tb1.objects.get(id=1)
obj.c1 = '111'
obj.save()                         # 修改单条数据

extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)
  Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,))
  Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
  Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])
  Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])

from django.db.models import F
  models.Tb1.objects.update(num=F('num')+1)

  Q(nid__gt=10)
  Q(nid=8) | Q(nid__gt=10)
  Q(Q(nid=8) | Q(nid__gt=10)) & Q(caption='root')

  con = Q()
  q1 = Q()
  q1.connector = 'OR'
  q1.children.append(('id', 1))
  q1.children.append(('id', 10))
  q1.children.append(('id', 9))
  q2 = Q()
  q2.connector = 'OR'
  q2.children.append(('c1', 1))
  q2.children.append(('c1', 10))
  q2.children.append(('c1', 9))
  con.add(q1, 'AND')
  con.add(q2, 'AND')
 
  models.Tb1.objects.filter(con)

# 条件查询
 # 条件可以是：参数，字典，Q

  model.tb.objects.all().select_related()
  model.tb.objects.all().select_related('外键字段')
  model.tb.objects.all().select_related('外键字段__外键字段')

# 第一次 获取所有用户表
# 第二次 获取用户类型表where id in (用户表中的查到的所有用户ID)
models.UserInfo.objects.prefetch_related('外键字段')

# 用于实现聚合group by查询
from django.db.models import Count, Avg, Max, Min, Sum
 v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id'))
# SELECT u_id, COUNT(ui) AS `uid` FROM UserInfo GROUP BY u_id
v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id')).filter(uid__gt=1)
# SELECT u_id, COUNT(ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1
v = models.UserInfo.objects.values('u_id').annotate(uid=Count('u_id',distinct=True)).filter(uid__gt=1)
# SELECT u_id, COUNT( DISTINCT ui_id) AS `uid` FROM UserInfo GROUP BY u_id having count(u_id) > 1

# 构造额外的查询条件或者映射，如：子查询
Entry.objects.extra(select={'new_id': "select col from sometable where othercol > %s"}, select_params=(1,))
Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])
Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])

# 倒序
 models.UserInfo.objects.all().order_by('-nid').reverse()
# 注：如果存在order_by，reverse则是倒序，如果多个排序则一一倒序

models.UserInfo.objects.defer('username','id')
或
models.UserInfo.objects.filter(...).defer('username','id')
# 映射中排除某列数据

# 仅取某个表中的数据
models.UserInfo.objects.only('username','id')
或
models.UserInfo.objects.filter(...).only('username','id')