mongo db Standard deviation calculation with map reduce

Calculate standard deviation in mongo

// sample data
{ "_id" : ObjectId("4caf19200d282159bf000001"), "date" : "2010-10-06", "seq" : "00:00:00,000", "method" : "getUserByScbeId", "duration" : 3 }
{ "_id" : ObjectId("4caf19200d282159bf000002"), "date" : "2010-10-06", "seq" : "00:00:00,116", "method" : "createTicket", "duration" : 116 }
{ "_id" : ObjectId("4caf19200d282159bf000003"), "date" : "2010-10-06", "seq" : "00:00:00,131", "method" : "getCollectionMetadata", "duration" : 11 }
{ "_id" : ObjectId("4caf19200d282159bf000004"), "date" : "2010-10-06", "seq" : "00:00:00,137", "method" : "getParticipation", "duration" : 6 }
{ "_id" : ObjectId("4caf19200d282159bf000005"), "date" : "2010-10-06", "seq" : "00:00:00,139", "method" : "updateSocialObjectModified", "duration" : 371 }
{ "_id" : ObjectId("4caf19200d282159bf000006"), "date" : "2010-10-06", "seq" : "00:00:00,143", "method" : "getUserByScbeId", "duration" : 4 }




// map reduce implementation

map = function() { 
	emit(this.method, { duration : this.duration, count : 1}); 
}

reduce = function(key,emits) { 
	var n = { count : 0, duration : 0, min : Number.MAX_VALUE, max : Number.MIN_VALUE };

	for(var i in emits) {
		n.count += emits[i].count;
		n.duration += emits[i].duration;
	}
	return n;
}

fin = function(key, value) {
	value.avg = value.duration / value.count;
	res2 = db.logs.find({method:key});
	sum2 = 0;
	res2.forEach( function(v) { 
		if(v.duration > value.max ) {
			value.max = v.duration;
		}
		if(v.duration < value.min ) {
			value.min = v.duration;
		}

		tmp1 = (v.duration - value.avg);
		sum2 += tmp1*tmp1;
	} );

	value.stdiv = Math.sqrt(sum2/value.count);

	return value;
}

res = db.logs.mapReduce(map, reduce, {finalize : fin});
db[res.result].find();

db seems to be undefined in the finalize function. At least in version 1.8.3.

you can no longer access db in the finalize function...

any other suggestions of doing it without the db?

working on it...

actually, what i did is to calculate the deviation in the reduce function:

function Reduce(key, values) {
var ret = { count: 0, sum: 0 };

values.forEach(function(v){
ret.count += v.count;
ret.sum += v.duration;
});

ret.avg = ret.count == 0 ? 0 : ret.sum / ret.count;

sum2 = 0;
values.forEach(function(v){
tmp1 = (v.duration - ret.avg);
sum2 += tmp1 * tmp1;
});

ret.stdiv = ret.count == 0 ? 0 : Math.sqrt(sum2 / ret.count);

return ret;
}

I made another, similar solution also using the reduce function. It does the calculation in a single pass.
Look in my fork.

like :)