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
}