Mongo Snippet

aggregate.js

/*
> for(var i = 0; i < 100000; i++ ) {
    db.sample.insert({"value": Math.random(1000) + 1, "date": new Date() });
  }

> db.sample.findOne()
{
	"_id" : ObjectId("5560cd3bf32e638da3683b38"),
	"value" : 1.3218692857772112,
	"tag" : "pressure"
}
*/

// select tagId _id, avg(value) avgAmount from sample
// group by tag
db.sample.aggregate( 
[  
	{
		$group: {   
			_id: "$tag", 
			avgAmount: { $avg: "$value"} 
		}
	}	 
]);

// select tagId _id, avg(value) avgAmount from sample
// group by tag
// order by tagId
db.historyMeters.aggregate([
    {
        $group: {'_id': '$tagId', 'avgMeter': {$avg: '$value'}}
    },
    {
        $sort: {'_id': 1}
    }
])

// select tagId, avg(value) avgAmount from sample
// group by tag
// order by tagId
db.historyMeters.aggregate([
    {
        $group: {'_id': '$tagId', 'avgMeter': {$avg: '$value'}}
    },
    {
        $sort: {'_id': 1}
    },
    {
       $project: {'_id': 0, 'avgMeter': 1, 'tagId': '$_id'}
    }
])


// select tagId _id, avg(value) avgAmount from sample
// where tagId = 1
// group by tag
db.historyMeters.aggregate([
    {
        $match: {'tagId': 1}
    },
    {
        $group: {'_id': '$tagId', 'avgMeter': {$avg: '$value'}}
    }
])

//average per hour
db.historyMeters.aggregate(
   [
     {
         $group: {
            '_id': { 
                'tagId': "$tagId", 'year': {$year: '$dt'}, 'month': {$month: '$dt'}, 'day': {$dayOfMonth: '$dt'}, 'hour': {$hour: '$dt'}
             }, 
            'avgMeter': {$avg: '$value'}
        }
     },
     {
         $sort: {'_id.tagId': 1, '_id.year': 1, '_id.month': 1, '_id.day': 1, '_id.hour': 1}
     },
     {
         $project: {
            '_id': 0, 
            'dt': { 'year': '$_id.year', 'month': '$_id.month', 'day': '$_id.day', 'hour': '$_id.hour'},
            'tagId': '$_id.tagId',
             'avgMeter': 1
         }
     }
   ]
)

benchmark.js

var start = new Date();

// Some code that takes some time
db.sample.aggregate( 
[  
	{
		$group: {   
			_id: "$tag", 
			avgAmount: { $avg: "$value"} 
		}
	}	 
]);

var end = new Date();
var secondsElapsed = (end - start) / 1000;