> for (var i = 0; i < 10000000; ++i){ db.uniques.insert({ dim0: Math.floor(Math.random()*1000000) });}
> db.uniques.findOne()
{ "_id" : ObjectId("51d3c386acd412e22c188dec"), "dim0" : 570859 }
> db.uniques.ensureIndex({dim0: 1})

> db.runCommand({ mapreduce: "uniques", map: function () { emit(this.dim0, 1); }, reduce: function (key, values) { return Array.sum(values); }, out: "mrout", sort: {dim0: 1} })
{
        "result" : "mrout",
        "timeMillis" : 192589,
        "counts" : {
                "input" : 10000000,
                "emit" : 10000000,
                "reduce" : 1000372,
                "output" : 999961
        },
        "ok" : 1
}

> var res = db.runCommand({splitVector: "test.uniques", keyPattern: {dim0: 1}, maxChunkSizeBytes: 32 *1024 * 1024 })
> var keys = res.splitKeys
> keys.length
39
> var mapred = function(min, max) { return db.runCommand({ mapreduce: "uniques", map: function () { emit(this.dim0, 1); }, reduce: function (key, values) { return Array.sum(values); }, out: "mrout" + min, sort: {dim0: 1}, query: { dim0: { $gte: min, $lt: max } } }) }
> var numThreads = 4
> var inc = Math.floor(keys.length / numThreads) + 1
> threads = []; for (var i = 0; i < numThreads; ++i) { var min = (i == 0) ? 0 : keys[i * inc].dim0; var max = (i * inc + inc >= keys.length) ? MaxKey : keys[i * inc + inc].dim0 ; print("min:" + min + " max:" + max); var t = new ScopedThread(mapred, min, max); threads.push(t); t.start() }
min:0 max:274736
min:274736 max:524997
min:524997 max:775025
min:775025 max:{ "$maxKey" : 1 }
connecting to: test
connecting to: test
connecting to: test
connecting to: test
> for (var i in threads) { var t = threads[i]; t.join(); printjson(t.returnData()); }
{ 
        "result" : "mrout0",
        "timeMillis" : 205790,
        "counts" : {
                "input" : 2750002,
                "emit" : 2750002,
                "reduce" : 274828,
                "output" : 274723
        },
        "ok" : 1
}
...

> var mapred = function(min, max) { return db.runCommand({ mapreduce: "uniques", map: function () { emit(this.dim0, 1); }, reduce: function (key, values) { return Array.sum(values); }, out: { replace: "mrout" + min, db: "mrdb" + min }, sort: {dim0: 1}, query: { dim0: { $gte: min, $lt: max } } }) }
> threads = []; for (var i = 0; i < numThreads; ++i) { var min = (i == 0) ? 0 : keys[i * inc].dim0; var max = (i * inc + inc >= keys.length) ? MaxKey : keys[i * inc + inc].dim0 ; print("min:" + min + " max:" + max); var t = new ScopedThread(mapred, min, max); threads.push(t); t.start() }
min:0 max:274736
min:274736 max:524997
min:524997 max:775025
min:775025 max:{ "$maxKey" : 1 }
connecting to: test
connecting to: test
connecting to: test
connecting to: test
> for (var i in threads) { var t = threads[i]; t.join(); printjson(t.returnData()); }
{ 
        "result" : {
                "db" : "mrdb0",
                "collection" : "mrout0"
        },
        "timeMillis" : 108790,
        "counts" : {
                "input" : 2750002,
                "emit" : 2750002,
                "reduce" : 274828,
                "output" : 274723
        },
        "ok" : 1
}
...