MongoDB Performance: Group vs. Find vs. MapReduce

Lately, I've been playing a lot with MongoDB's group command. It's a powerful command that allows you to write some really interesting queries quickly. Here's an example of one that calculates "Tags" on a blog site:

	app.get('/find/other', function(req, res) {
	var reduce = function(obj, prev) {
	obj.tags.forEach(function(tag) {
	prev.tagCounts[tag] = prev.tagCounts[tag] ? prev.tagCounts[tag] + 1 : 1;
	})
	};
	db.reviews.group({}, {}, {tagCounts:{}}, reduce, function(err, result) {
	// get the tags
	var tags = result[0].tagCounts,
	sorted = [],
	scale = 0;
	// sort the tags
	for (tag in tags) {
	sorted.push({
	tag: tag,
	count:tags[tag],
	});
	}
	// sort the tags
	sorted.sort(function(a, b) {
	return a.count > b.count;
	});
	// figure out the scale and apply it
	scale = sorted[sorted.length - 1].count;
	sorted.forEach(function(tag) {
	tag.num = (tag.count / scale).toFixed(1);
	});
	// randomize
	sorted.sort(function() {
	return 0.5 - Math.random()
	})
	// render tags page
	res.render('find/tags.html', {
	tags: sorted
	});
	});
	});

view raw tags.js hosted with ❤ by GitHub

In my tests it also proved to be about 4x faster than a similar MapReduce, however, it comes with a severe cost: It blocks all reading from the collection. This is a huge problem and basically makes it worthless for doing serious queries on a database with say hundreds of thousands of users like I have in my day job. From what I can tell finds, distincts, and mapReduces don't block and some combination of those provide non-blocking alternatives.

Here's a simple map reduce example:

	// groups users by type
	var map = function() {
	emit(this.type, 1)
	}
	var reduce = function(key, values) {
	var result = 0
	values.forEach(function(value) {
	result += value
	})
	return result
	}
	var options = {out:{inline:1}}
	db.users.mapReduce(map, reduce, options)

view raw mr.js hosted with ❤ by GitHub

One more note, I recently read MongoDB: The Definitive Guide, which had a lot of examples and clarification that are not readily available in the online documentation. I highly suggest checking it out!