(Django)对object.all()等大量数据的QuerySet限制内存使用

问题

在django的使用中，经常会出现大量数据的遍历操作，或者是对大量数据进行遍历迁移跟新，比如

for user in User.objects.all():
user.A = user.B
User.B = None

等种种情况。

在本地开发环境中QuerySet对象最初具有非常小的内存占用，随着业务量的增长QuerySet对象在我遍历它们时缓存每个model_instance，all()返回的QuerySet会越来越来，可能最终耗尽内存，被托管服务提供商杀死线程。

解决方法:

import copy
from decimal import Decimal

class MemorySavingQuerysetIterator(object):
def __init__(self, queryset, max_obj_num=1000):
self._base_queryset = queryset
self._generator = self._setup()
self.max_obj_num = max_obj_num

def _setup(self):
for i in xrange(0, self._base_queryset.count(), self.max_obj_num):
# By making a copy of of the queryset and using that to actually
# access the objects we ensure that there are only `max_obj_num`
# objects in memory at any given time
smaller_queryset = copy.deepcopy(self._base_queryset
)[i:i + self.max_obj_num]
# logger.debug('Grabbing next %s objects from DB' % self.max_obj_num)
for obj in smaller_queryset.iterator():
yield obj

def __iter__(self):
return self

def next(self):
return self._generator.next()

调用：

Users = User.objects.all()
for user in MemorySavingQuerysetIterator(users, 100):
Pass

python mysql原生操作

import MySQLdb

class QuerySetIterator(object):
def __init__(self, cursor, query, max_num):
self.query = query
self.max_num = max_num
self._cursor = cursor
self._generator = self._setup()

def _setup(self):
for i in xrange(0, 90000000, self.max_num):
new_query = "{query} limit {limit} offset {offset}".format(
query=self.query, limit=self.max_num, offset=i
)
self._cursor.execute(new_query)
result = self._cursor.fetchall()
if not result:
break
for obj in result:
yield obj

def __iter__(self):
return self

def next(self):
return self._generator.next()

class TestModel(object):
db = MySQLdb.connect("localhost", "root", "123456", "test")
cursor = db.cursor()

def __init__(self, tb_name, max_num=100):
self.tb_name = tb_name
self.max_num = max_num
self._query_sql_tpl = "select * from {tb_name}".format(tb_name=tb_name)

def query_all(self, query_sql=None):
if not query_sql:
query_sql = self._query_sql_tpl
return QuerySet(self.cursor, query_sql, self.max_num)

test = TestModel('test')
result = test.query_all()

for obj in result:
print obj