MongoDB 聚合报错
聚合框架它是数据聚合的一个新框架，其概念类似于数据处理的管道。 每个文档通过一个由多个节点组成的管道，每个节点有自己特殊的
聚合框架它是数据聚合的一个新框架，其概念类似于数据处理的管道。 每个文档通过一个由多个节点组成的管道，每个节点有自己特殊的功能（分组、过滤等），文档经过管道处理后，最后输出相应的结果。
管道基本的功能有两个：
一是对文档进行“过滤”，也就是筛选出符合条件的文档;
二是对文档进行“变换”，也就是改变文档的输出形式。
errmsg:Exceeded memory limit for $group,but didn't allow external Sort.Pass allowDiskuse:true
错误详细信息：
我这次用的表的数据量是：大约有四百万数据
我要查的是：用户接受信息最多的前五个用户
写完分析语句，严格按照语法写的；
既然出现问题，我们来一下Debug（咱们码农有力量）
如果管道没有给出预期结果，就需要进行调试，调试时，可以先只指定第一个管道操作符，如果这样得到了预期结果，那就再指定第二个管道操作符，以上面的例子说，先试着$project 操作符进行聚合，如果这个操作符结果是有效的，就在添加 $group 操作符，
如果结果还是有效的，就再添加 $sort,最后再添加 $limit 操作符，这样就可以逐步定位到造成问题的操作符了。
逐步调试发现没问题 不是语法的错误
麻蛋，，原因是聚合的结果必须要限制在16M以内操作，（mongodb支持的最大影响信息的大小），否则必须放在磁盘中做缓存（allowDiskUse=True）。
最后把要统计的数据给搞出来了，喜大普奔。
更多MongoDB相关内容可以看看以下的有用链接：
MongoDB 3.0 正式版发布下载
CentOS编译安装MongoDB
CentOS 编译安装 MongoDB与mongoDB的php扩展
CentOS 6 使用 yum 安装MongoDB及服务器端配置
Ubuntu 13.04下安装MongoDB2.4.3
MongoDB入门必读(概念与实战并重)
Ubunu 14.04下MongoDB的安装指南
《MongoDB 权威指南》(MongoDB: The Definitive Guide)英文文字版[PDF]
Nagios监控MongoDB分片集群服务实战
基于CentOS 6.5操作系统搭建MongoDB服务
MongoDB 的详细介绍：请点这里
MongoDB 的下载地址：请点这里
本文永久更新链接地址：
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