本文中的实验仅限于特定数据库和特定框架。不同数据库，数据库服务器的性能，甚至同一个数据库的不同配置都会影响到同一段代码的性能。具体情况请在自己的生产环境进行测试。
这里（stackoverflow）有一篇关于使用Django随机获取记录的讨论。主要意思是说
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Record.objects.order_by('?')[:2]
这样获取2个记录会导致性能问题，原因如下：
“?对于有着相当多数量记录的表来说，这种方法异常糟糕。这会导致一个 ORDER BY RAND() 的SQL查询。举个栗子，这里是MYSQL是如何处理这个查询的（其他数据库的情况也差不多），想象一下当一个表有十亿行的时候会怎样：
- 为了完成ORDER BY RAND() ，需要一个RAND()列来排序为了有RAND()列，需要一个新表，因为现有的表没有这个列。为了这个新表，mysql建立了一个带有新列的，新的临时表，并且将已有的一百万行数据复制进去。当其新建完了，他如你所要求的，为每一行运行RAND()函数来填上这个值。是的，你派mysql创建一百万个随机数，这要点时间：）几个小时或几天后，当他干完这活，他要排序。是的，你排mysql去排序一个一百万行的，最糟糕的表（说他最糟糕是因为排序的键是随机的）。几天或者几星期后，当排序完了，他忠诚地将你实际需要的可怜的两行抓出来返回给你。做的好。；）
注意：只是稍微说一句，得注意到mysql一开始会试着在内存中创建临时表。当内存不够了，他将会把所有东西放在硬盘上，所以你会因为近乎于整个过程中的I/O瓶颈而雪上加霜。
怀疑者可以去看看python代码引起的查询语句，确认是ORDER BY RAND()， 然后去Google下”order by rand()”（带上引号）。
一个更好的方式是将这个耗费严重的查询换成3个耗费更轻的：
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last = MyModel.objects.count() - 1# 这是一个获取两个不重复随机数的简单方法? index1 = randint(0, last)? index2 = randint(0, last - 1)? if index2 == index1:?? ????index2 = last? MyObj1 = MyModel.objects.all()[index1]? MyObj2 = MyModel.objects.all()[index2]
”
如上Manganeez所说的方法，相应的获取n条记录的代码应该如下：
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sample = random.sample(xrange(Record.objects.count()),n)? result = [Record.objects.all()[i] for i in sample]
基于Python代码应该简洁优雅的想法，如上的代码似乎可以写成：
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result = random.sample(Record.objects.all(),n)
就性能问题，请教了stackoverflow上的大神 （虽然被踩和被教育了=。=）
“Record.objects.count() 将被转换成一个相当轻量级的SQL请求：
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SELECT COUNT(*) FROM TABLE
Record.objects.all()[0]也会被转换成一个十分轻量级的SQL请求
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SELECT * FROM TABLE LIMIT 1
Querying all 是一个耗费十分严重的请求
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SELECT * FROM TABLE
通常情况下Django会不显示其他的结果，这样你不会真正的获取到所有的记录。
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SELECT * FROM table LIMIT 20;? // or something similar
任何时候你将一个Queryset转换成list的时候，将是资源消耗严重的时候。
如果我没错的话，在这个例子里，sample方法将把Queryset转换成list。
这样如果你result = random.sample(Record.objects.all(),n) 这样做的话，全部的Queryset将会转换成list，然后从中随机选择。
想象一下如果你有十亿行的数据。你是打算把它存储在一个有百万元素的list中，还是愿意一个一个的query？
”
在上边Yeo的回答中，freakish回复道：“.count的性能是基于数据库的。而Postgres的.count为人所熟知的相当之慢。”
某人说过，要知道梨子的滋味，就得变革梨子，亲口尝一尝。
测试环境：
Win8.1 pro x64Wampserver2.4-x64 (apache2.4.4 mysql5.6.12 php5.4.12)Python2.7.5Django1.4.6
在一个已有的测试project中新建一个app，数据库是MYSQL：
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D:\PyWorkspace\DjangoTest>python manage.py startapp randomrecords
在models.py中添加模型：
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class Record(models.Model):? ????"""docstring for Record"""????id = models.AutoField(primary_key = True)? ????content = models.CharField(max_length = 16)? ????def __str__(self):? ????????return "id:%s content:%s" % (self.id, self.content)? ????def __unicode__(self):? ????????return u"id:%s content:%s" % (self.id, self.content)
添加一万行数据：
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D:\PyWorkspace\DjangoTest>python manage.py syncdb? Creating tables ...? Creating table randomrecords_record? Installing custom SQL ...? Installing indexes ...? Installed 0 object(s) from 0 fixture(s)? D:\PyWorkspace\DjangoTest>python manage.py shell? Python 2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit (AMD64)] on win? 32Type "help", "copyright", "credits" or "license" for more information.? (InteractiveConsole)? >>> from randomrecords.models import Record? >>> for i in xrange(10000):? ...?? Record.objects.create(content = 'c of %s' % i).save()
15分钟以后我得到了这个MYSQL表。真的，不骗你，真的是15分钟。看了记录才知道 每次save都要调用一次insert和一次update。。。。下次一定用SQL语句初始化。。。。
先写了个脚本 在manage.py shell中调用了下 结果让我震惊了。我表示不敢相信 又写了view 并在settings.py中添加了显示SQL Query语句的log
这里是写的view：
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def test1(request):? ????start = datetime.datetime.now()? ????result = Record.objects.order_by('?')[:20]? ????l = list(result) # Queryset是惰性的，强制将Queryset转为list? ????end = datetime.datetime.now()? ????return HttpResponse("time:? %s" % (end-start).microseconds/1000))? def test2(request):? ????start = datetime.datetime.now()? ????sample = random.sample(xrange(Record.objects.count()),20)? ????result = [Record.objects.all()[i] for i in sample]? ????l = list(result)? ????end = datetime.datetime.now()? ????return HttpResponse("time:? %s" % (end-start)? def test3(request):? ????start = datetime.datetime.now()? ????result = random.sample(Record.objects.all(),20)? ????l = list(result)? ????end = datetime.datetime.now()? ????return HttpResponse("time:? %s" % (end-start)
运行结果如下，第一行是页面显示的时间，后边是Queryset实际调用的SQL语句
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test1：? time: 0:00:00.012000? (0.009) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` ORDER BY RAND() LIMIT 20; args=()? [05/Dec/2013 17:48:19] "GET /dbtest/test1 HTTP/1.1" 200 775? test2:? time: 0:00:00.055000? (0.002) SELECT COUNT(*) FROM `randomrecords_record`; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 6593; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 2570; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 620; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 5814; args=()? (0.003) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 6510; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3536; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3362; args=()? (0.003) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 8948; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 7723; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 2374; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 8269; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4370; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 6953; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1441; args=()? (0.000) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 772; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4323; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 8139; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 7441; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1306; args=()? (0.001) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 5462; args=()? [05/Dec/2013 17:50:34] "GET /dbtest/test2 HTTP/1.1" 200 777? test3:? time: 0:00:00.156000? (0.032) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record`; args=()? [05/Dec/2013 17:51:29] "GET /dbtest/test3 HTTP/1.1" 200 774
令人难以置信的，在10000行的MYSQL表中 方法1的效率是最高的。无论是结果上看（12ms）还是SQL语句的运行时间上看（9ms）方法1甩了其他方法一大截
即便数据量增加到21万，方法1也会比其他两种方法快：
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time: 98? (0.094) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` ORDER BY RAND() LIMIT 20; args=()? [05/Dec/2013 19:18:59] "GET /dbtest/test1 HTTP/1.1" 200 14? time: 0:00:00.668000? //这里没有注意到 掉了一行count语句? (0.045) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 176449; args=()? (0.016) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 68082; args=()? (0.036) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 145571; args=()? (0.033) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 111029; args=()? (0.043) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 169675; args=()? (0.046) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 186234; args=()? (0.043) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 167233; args=()? (0.015) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 54404; args=()? (0.036) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 140395; args=()? (0.004) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 13128; args=()? (0.039) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 153695; args=()? (0.034) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 131863; args=()? (0.021) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 82785; args=()? (0.015) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 57253; args=()? (0.021) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 77836; args=()? (0.049) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 199567; args=()? (0.002) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3867; args=()? (0.027) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 104470; args=()? (0.026) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 107058; args=()? (0.043) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 150979; args=()? [05/Dec/2013 19:21:33] "GET /dbtest/test2 HTTP/1.1" 200 15? time 0:00:00.781000? [05/Dec/2013 19:23:01] "GET /dbtest/test3 HTTP/1.1" 200 15? (0.703) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record`; args=()? [05/Dec/2013 19:23:06] "GET /dbtest/test3 HTTP/1.1" 200 15
数据量再次提升至百万级别 1066768条数据
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time:?? 0:00:02.197000? (2.193) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` ORDER BY RAND() LIMIT 20; args=()? [05/Dec/2013 20:00:55] "GET /dbtest/test1 HTTP/1.1" 200 26? time:?? 0:00:02.659000? (0.204) SELECT COUNT(*) FROM `randomrecords_record`; args=()? (0.180) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 703891; args=()? (0.038) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 156668; args=()? (0.013) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 50742; args=()? (0.031) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 121107; args=()? (0.033) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 130565; args=()? (0.017) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 66225; args=()? (0.234) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 922479; args=()? (0.267) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1027166; args=()? (0.189) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 765499; args=()? (0.009) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 31569; args=()? (0.233) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 934055; args=()? (0.264) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1052741; args=()? (0.155) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 621692; args=()? (0.014) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 52388; args=()? (0.199) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 759669; args=()? (0.170) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 655598; args=()? (0.035) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 139709; args=()? (0.228) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 919480; args=()? (0.104) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 422051; args=()? (0.017) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 67549; args=()? [05/Dec/2013 20:00:45] "GET /dbtest/test2 HTTP/1.1" 200 26? time:?? 0:00:19.651000? (3.645) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record`; args=()? [05/Dec/2013 20:02:50] "GET /dbtest/test3 HTTP/1.1" 200 26
第三种方法所用时间长到令人无法接受（19.65秒）。但有意思的是 SQL语句所花费的时间“只有”3.6秒。而大部分的时间都用在python上了。
既然第二种方法和第三种方法都需要random.sample 一个百万个数据的list，那就是说，有大量的时间花费在将SELECT到的结果转化为django对象的过程中了。
此后将不再测试第三种方法
最后，数据量增加到5,195,536个
随着表中数据行数的增加，两个方法的所用的时间都到了一个完全不能接受的程度。两种方法所用的时间也几乎相同。
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time:?? 0:00:22.278000? (22.275) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FR? OM `randomrecords_record` ORDER BY RAND() LIMIT 20; args=()? [05/Dec/2013 21:46:33] "GET /dbtest/test1 HTTP/1.1" 200 26? time:?? 0:00:33.319000? (1.393) SELECT COUNT(*) FROM `randomrecords_record`; args=()? (3.201) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4997880; args=()? (1.229) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 2169311; args=()? (0.445) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1745307; args=()? (1.306) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3233861; args=()? (1.881) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3946647; args=()? (1.624) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3534377; args=()? (1.068) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1684337; args=()? (0.902) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 2607361; args=()? (2.938) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4872494; args=()? (0.493) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 851494; args=()? (3.275) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 5182414; args=()? (0.946) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1684670; args=()? (0.701) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1819730; args=()? (0.915) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1626221; args=()? (1.809) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 3638682; args=()? (3.237) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4801027; args=()? (1.187) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1955843; args=()? (2.736) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 4835733; args=()? (1.705) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 2756641; args=()? (0.286) SELECT `randomrecords_record`.`id`, `randomrecords_record`.`content` FRO? M `randomrecords_record` LIMIT 1 OFFSET 1117426; args=()
值得注意的是，Mysql数据库有一个特点是，对于一个大表，OFFSET越大，查询时间越长。或许有其他方法可以在offset较大的时候加快select的速度，然而django明显没有做到。如果能够减少这种消耗，方法2明显会优于方法1。
附上三种方法数据量和SQL时间/总时间的数据图表：
最后总结，Django下，使用mysql数据库，数据量在百万级以下时，使用
1
Record.objects.order_by('?')[:2]
来获取随机记录序列，性能不会比
12
sample?=?random.sample(xrange(Record.objects.count()),n) ?result?=?[Record.objects.all()[i])?for?i?in?sample]
差。
原文地址：使用Django从数据库中随机取N条记录的不同方法及其性能实测, 感谢原作者分享。 经受雨，面对另一个轮回。