Django的model查詢操做 與 查詢性能優化

Django的model查詢操做 與 查詢性能優化

1 如何 在作ORM查詢時 查看SQl的執行狀況

(1) 最底層的 django.db.connection

在 django shell 中使用  python manage.py shell

>>> from django.db import connection
>>> Books.objects.all()
>>> connection.queries    ## 能夠查看查詢時間
[{'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 強化）


    這樣每次查詢都會 有sql 輸出

    >>>  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查詢操做 以及優化

基本操做

增

models.Tb1.objects.create(c1='xx', c2='oo')  增長一條數據，能夠接受字典類型數據 **kwargs

obj = models.Tb1(c1='xx', c2='oo')
obj.save()

 查

models.Tb1.objects.get(id=123)         # 獲取單條數據，不存在則報錯（不建議）
models.Tb1.objects.all()               # 獲取所有
models.Tb1.objects.filter(name='seven') # 獲取指定條件的數據
models.Tb1.objects.exclude(name='seven') # 獲取指定條件的數據

 刪

models.Tb1.objects.filter(name='seven').delete() # 刪除指定條件的數據

 改
models.Tb1.objects.filter(name='seven').update(gender='0')  # 將指定條件的數據更新，均支持 **kwargs
obj = models.Tb1.objects.get(id=1)
obj.c1 = '111'
obj.save()                                                 # 修改單條數據

查詢簡單操做

獲取個數

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

大於，小於

    models.Tb1.objects.filter(id__gt=1)              # 獲取id大於1的值
    models.Tb1.objects.filter(id__gte=1)              # 獲取id大於等於1的值
    models.Tb1.objects.filter(id__lt=10)             # 獲取id小於10的值
    models.Tb1.objects.filter(id__lte=10)             # 獲取id小於10的值
    models.Tb1.objects.filter(id__lt=10, id__gt=1)   # 獲取id大於1 且 小於10的值

in

    models.Tb1.objects.filter(id__in=[11, 22, 33])   # 獲取id等於十一、2二、33的數據
    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大小寫不敏感
    models.Tb1.objects.exclude(name__icontains="ven")

range

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

其餘相似

    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正則匹配，iregex 不區分大小寫

    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)

查詢複雜操做

FK foreign key 使用的緣由:

約束
節省硬盤

可是多表查詢會下降速度，大型程序反而不使用外鍵，而是用單表(約束的時候，經過代碼判斷)

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)

# 條件查詢
    # 條件能夠是：參數，字典，Q

select_related(self, *fields)

性能相關：表之間進行join連表操做，一次性獲取關聯的數據。
     model.tb.objects.all().select_related()
     model.tb.objects.all().select_related('外鍵字段')
     model.tb.objects.all().select_related('外鍵字段__外鍵字段')

prefetch_related(self, *lookups)

性能相關：多表連表操做時速度會慢，使用其執行屢次SQL查詢  在內存中作關聯，而不會再作連表查詢
            # 第一次 獲取全部用戶表
            # 第二次 獲取用戶類型表where id in (用戶表中的查到的全部用戶ID)
            models.UserInfo.objects.prefetch_related('外鍵字段')

annotate(self, *args, **kwargs)

# 用於實現聚合group by查詢

    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)

# 構造額外的查詢條件或者映射，如：子查詢

        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：若是存在order_by，reverse則是倒序，若是多個排序則一一倒序

下面兩個 取到的是對象，而且注意 取到的對象能夠 獲取其餘字段（這樣會再去查找該字段下降性能
defer(self, *fields):

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

only(self, *fields):

# 僅取某個表中的數據
         models.UserInfo.objects.only('username','id')
         或
         models.UserInfo.objects.filter(...).only('username','id')

執行原生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'，指定數據庫

    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)