详解Django的model查询操作与查询性能优化

1 如何 在做ORM查询时 查看SQl的执行情况

(1) 最底层的 django.db.connection

在 django shell 中使用  python manage.py shell

>>> from django.db import connection
>>> Books.objects.all()
>>> connection.queries  ## 可以查看查询时间
[{'sql': 'SELECT "testsql_books"."id", "testsql_books"."name", "testsql_books"."author_id" FROM "testsql_books" LIMI
T 21', 'time': '0.002'}]

(2) django-extensions 插件

pip install django-extensions

 INSTALLED_APPS = (
    ...
    'django_extensions',
    ...
    )

在 django shell 中使用  python manage.py shell_plus  --print-sql （extensions 强化）

这样每次查询都会 有sql 输出

>>> from testsql.models import Books
>>> Books.objects.all()
  SELECT "testsql_books"."id", "testsql_books"."name", "testsql_books"."author_id" FROM "testsql_books" LIMIT 21

Execution time: 0.002000s [Database: default]

<QuerySet [<Books: Books object>, <Books: Books object>, <Books: Books object>]>

2 ORM查询操作 以及优化

基本操作

增

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()

 查

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

 删

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

 改
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

查询简单操作

获取个数

  models.Tb1.objects.filter(name='seven').count()

大于，小于

  models.Tb1.objects.filter(id__gt=1)       # 获取id大于1的值
  models.Tb1.objects.filter(id__gte=1)       # 获取id大于等于1的值
  models.Tb1.objects.filter(id__lt=10)       # 获取id小于10的值
  models.Tb1.objects.filter(id__lte=10)       # 获取id小于10的值
  models.Tb1.objects.filter(id__lt=10, id__gt=1)  # 获取id大于1 且 小于10的值

in

  models.Tb1.objects.filter(id__in=[11, 22, 33])  # 获取id等于11、22、33的数据
  models.Tb1.objects.exclude(id__in=[11, 22, 33]) # not in

isnull
  Entry.objects.filter(pub_date__isnull=True)

contains

  models.Tb1.objects.filter(name__contains="ven")
  models.Tb1.objects.filter(name__icontains="ven") # icontains大小写不敏感
  models.Tb1.objects.exclude(name__icontains="ven")

range

  models.Tb1.objects.filter(id__range=[1, 2])  # 范围bettwen and

其他类似

  startswith，istartswith, endswith, iendswith,

order by

  models.Tb1.objects.filter(name='seven').order_by('id')  # asc
  models.Tb1.objects.filter(name='seven').order_by('-id')  # desc

group by--annotate

  from django.db.models import Count, Min, Max, Sum
  models.Tb1.objects.filter(c1=1).values('id').annotate(c=Count('num'))
  SELECT "app01_tb1"."id", COUNT("app01_tb1"."num") AS "c" FROM "app01_tb1" WHERE "app01_tb1"."c1" = 1 GROUP BY "app01_tb1"."id"

limit 、offset

  models.Tb1.objects.all()[10:20]

regex正则匹配，iregex 不区分大小写

  Entry.objects.get(title__regex=r'^(An?|The) +')
  Entry.objects.get(title__iregex=r'^(an?|the) +')

date

  Entry.objects.filter(pub_date__date=datetime.date(2005, 1, 1))
  Entry.objects.filter(pub_date__date__gt=datetime.date(2005, 1, 1))

year

  Entry.objects.filter(pub_date__year=2005)
  Entry.objects.filter(pub_date__year__gte=2005)

month

  Entry.objects.filter(pub_date__month=12)
  Entry.objects.filter(pub_date__month__gte=6)

day

  Entry.objects.filter(pub_date__day=3)
  Entry.objects.filter(pub_date__day__gte=3)

week_day

  Entry.objects.filter(pub_date__week_day=2)
  Entry.objects.filter(pub_date__week_day__gte=2)

hour

  Event.objects.filter(timestamp__hour=23)
  Event.objects.filter(time__hour=5)
  Event.objects.filter(timestamp__hour__gte=12)

minute

  Event.objects.filter(timestamp__minute=29)
  Event.objects.filter(time__minute=46)
  Event.objects.filter(timestamp__minute__gte=29)

second

  Event.objects.filter(timestamp__second=31)
  Event.objects.filter(time__second=2)
  Event.objects.filter(timestamp__second__gte=31)

查询复杂操作

FK foreign key 使用的原因:

• 约束
• 节省硬盘

但是多表查询会降低速度，大型程序反而不使用外键，而是用单表(约束的时候，通过代码判断)

extra

  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'])

F

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

Q

  方式一：
  Q(nid__gt=10)
  Q(nid=8) | Q(nid__gt=10)
  Q(Q(nid=8) | Q(nid__gt=10)) & Q(caption='root')

  方式二：
  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)

exclude(self, *args, **kwargs)

  # 条件查询
  # 条件可以是：参数，字典，Q

select_related(self, *fields)

   性能相关：表之间进行join连表操作，一次性获取关联的数据。
   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')

prefetch_related(self, *lookups)

  性能相关：多表连表操作时速度会慢，使用其执行多次SQL查询 在内存中做关联，而不会再做连表查询
      # 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')

annotate(self, *args, **kwargs)

# 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

extra(self, select=None, where=None, params=None, tables=None, order_by=None, select_params=None)

 # 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'])

reverse(self):

# 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

下面两个 取到的是对象，并且注意 取到的对象可以 获取其他字段（这样会再去查找该字段降低性能
defer(self, *fields):

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

only(self, *fields):

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

执行原生SQL

1.connection
from django.db import connection, connections
cursor = connection.cursor()
# cursor = connections['default'].cursor()
django的settings中的db配置 ' default'，指定数据库
cursor.execute("""SELECT * from auth_user where id = %s""", [1])
row = cursor.fetchone()

2 .extra
Entry.objects.extra(select={'new_id': "select id from tb where id > %s"}, select_params=(1,), order_by=['-nid'])

3 . raw
name_map = {'a':'A','b':'B'}
models.UserInfo.objects.raw('select * from xxxx',translations=name_map)

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