[英]Efficient way to query in a for loop in Google App Engine?
在GAE文档中,它指出:
因为每个get()或put()操作都调用一个单独的远程过程调用(RPC),所以在循环内发出许多这样的调用是一次性处理实体或键集合的低效率方法。
谁知道我的代码中还有多少其他效率低下的问题,所以我想尽可能地减少效率。 目前,我确实有一个for循环,其中每个迭代都有一个单独的查询。 假设我有一个用户,一个用户有朋友。 我想为用户的每个朋友获取最新更新。 因此,我所拥有的是该用户好友的数组:
for friend_dic in friends:
email = friend_dic['email']
lastUpdated = friend_dic['lastUpdated']
userKey = Key('User', email)
query = ndb.gql('SELECT * FROM StatusUpdates WHERE ANCESTOR IS :1 AND modifiedDate > :2', userKey, lastUpdated)
qit = query.iter()
while (yield qit.has_next_async()):
status = qit.next()
status_list.append(status.to_dict())
raise ndb.Return(status_list)
有没有更有效的方法来执行此操作,也许以某种方式将所有这些批处理成一个查询?
尝试查看NDB的地图功能: https : //developers.google.com/appengine/docs/python/ndb/queryclass#Query_map_async
示例(假设您将朋友关系保留在单独的模型中,在本示例中,我假设使用“ Relationships
模型):
@ndb.tasklet
def callback(entity):
email = friend_dic['email']
lastUpdated = friend_dic['lastUpdated']
userKey = Key('User', email)
query = ndb.gql('SELECT * FROM StatusUpdates WHERE ANCESTOR IS :1 AND modifiedDate > :2', userKey, lastUpdated)
status_updates = yield query.fetch_async()
raise ndb.Return(status_updates)
qry = ndb.gql("SELECT * FROM Relationships WHERE friend_to = :1", user.key)
updates = yield qry.map_async(callback)
#updates will now be a list of status updates
更好地了解您的数据模型:
queries = []
status_list = []
for friend_dic in friends:
email = friend_dic['email']
lastUpdated = friend_dic['lastUpdated']
userKey = Key('User', email)
queries.append(ndb.gql('SELECT * FROM StatusUpdates WHERE ANCESTOR IS :1 AND modifiedDate > :2', userKey, lastUpdated).fetch_async())
for query in queries:
statuses = yield query
status_list.extend([x.to_dict() for x in statuses])
raise ndb.Return(status_list)
您可以使用ndb异步方法同时执行这些查询:
from google.appengine.ext import ndb
class Bar(ndb.Model):
pass
class Foo(ndb.Model):
pass
bars = ndb.put_multi([Bar() for i in range(10)])
ndb.put_multi([Foo(parent=bar) for bar in bars])
futures = [Foo.query(ancestor=bar).fetch_async(10) for bar in bars]
for f in futures:
print(f.get_result())
这将启动10个并发的Datastore Query RPC,并且总体延迟仅取决于最慢的延迟,而不是所有延迟的总和
另请参阅官方的ndb文档,以获取有关如何使用ndb异步API的更多详细信息。
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