解决MongoDB 排序超过内存限制的问题

对集合执行一个大排序操作（如聚合），出现以下错误：（测试版本：mongoDB 3.0.6）

> db.bigdata.aggregate(
 {$group : {_id : "$range", total : { $sum : 1 }}},
 {$sort : {total : -1}}
);
#...
 aggregate failed
 at Error (<anonymous>)
 at doassert (src/monGo/shell/assert.js:11:14)
 #...
 Error: command failed: {
 "errmsg" : "exception: Sort exceeded memory limit of 104857600 bytes, 
 but did not opt in to external sorting. Aborting operation. Pass allowDiskUse:true to opt in.",
 "code" : 16819,
 "ok" : 0
 }