Django的model查询操作 与 查询性能优化

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)