How to get an explain for a MongoDB count?

Mongo 3.0 introduced a new way of explaining non-cursor requests:

db.my_collection.explain().count()

See: http://docs.mongodb.org/manual/reference/method/db.collection.explain/#db.collection.explain

Based on https://jira.mongodb.org/browse/SERVER-14098, a new future version will support this in the format:

db.runCommand({    explain: {        count: 'collectionName',        query: {            foo: 'bar'        }    }})

I'm pretty sure that count(query) is short for find(query).count()--in other words, the explain is exactly the same. There's no specific counting optimization done, except maybe for a full collection count. For example, running a count on a non-indexed field for a range takes exactly the same amount of time as running a find.explain with the same range.

I wrote a function called timeCount that takes the average of the count function time, and then shows the explain output for comparison.

function timeCount(coll, query) {  var n = 5;  var total = 0;  for(var i = 0; i < n; i++) {    var start = new Date();    db[coll].find(query).count();    var end = new Date();    total += (end - start);    print("time[" + i + "]: " + (end - start) + "ms");  }  print("average time: " + (total / n));  var explain = db[coll].find(query).explain();  print("explain (from find): ");  for(e in explain) {    if(typeof explain[e] == "string" || typeof explain[e] == "number") {      print(e + ": " + explain[e]);    }  }}

The output looks like this:

> timeCount('test',{x:{$gt:5000}});time[0]: 1339mstime[1]: 1280mstime[2]: 1347mstime[3]: 1322mstime[4]: 1299msaverage time: 1317.4explain (from find): cursor: BtreeCursor x_1_y_1nscanned: 995062nscannedObjects: 995062n: 995062millis: 1390nYields: 0nChunkSkips: 0