使用MongoDB聚合框架获取数组大小直方图的最快方法 [英] Fastest way to get histogram of array sizes using MongoDB aggregation framework
问题描述
我正在尝试获取具有可变大小数组的记录数的列表.我想获取所有记录的数组大小分布,这样我就可以构建一个直方图:
I'm trying to get a list of the number of records that have arrays of varying size. I want to get the distribution of array sizes for all records so I can build a histogram like this:
| *
| *
documents | * *
| * * *
|_*__*__*___*__*___
2 5 6 23 47
Array Size
因此原始文档看起来像这样:
So the raw documents look something like this:
{hubs : [{stuff:0, id:6}, {stuff:1"}, .... ]}
{hubs : [{stuff:0, id:6}]}`
到目前为止,在此处我想出了
So far using the aggregation framework and some of the help here I've come up with
db.sitedata.aggregate([{ $unwind:'$hubs'},
{ $group : {_id:'$_id', count:{$sum:1}}},
{ $group : {_id:'$count', count:{$sum:1}}},
{ $sort : {_id: 1}}])
这似乎给了我想要的结果,但是速度不是很快.我想知道是否有我可以做的事情,可能不需要两个小组呼叫.这里的语法是错误的,但是我想做的是将计数值放在第一个_id字段中:
This seems to give me the results I want, but it's not very fast. I'm wondering if there is something I can do like this that may not need two group calls. The syntax is wrong here, but what I'm trying to do is put the count value in the first _id field:
db.sitedata.aggregate([{ $unwind:'$hubs'},
{ $group : {_id:{$count:$hubs}, count:1}},
{ $sort : { _id: 1 }}])
推荐答案
Now that 2.6 is out, aggregation framework supports a new array operator $size
which will allow you to $project
the array size without having to unwind and re-group.
db.sitedata.aggregate([{ $project:{ 'count': { '$size':'$hubs'} } },
{ $group : {_id:'$count', count:{$sum:1} } },
{ $sort : { _id: 1 } } ] )
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