Index optimization for mongodb aggregation framework

Question

I have a match-unwind-group-sort aggregation pipeline in mongo 2.4.4 and I need to speed up the aggregation.

The match operation consists of range queri

Answer 1

For the first question, yes, you can explain aggregates.

db.collection.runCommand("aggregate", {pipeline: YOUR_PIPELINE, explain: true})

For the second one, the indexes you create to optimize the range queries will also apply to the $match stage of the aggregation pipeline, if they occur at the beginning of the pipeline. So you are right to focus on index optimizations.

See Pipeline Operators and Indexes.

Update 2

More about aggregate and explain: on version 2.4 it is unreliable; on 2.6+ it does not provide query execution data. https://groups.google.com/forum/#!topic/mongodb-user/2LzAkyaNqe0

Update 1

Transcript of an aggregation explain on MongoDB 2.4.5.

$ mongo so
MongoDB shell version: 2.4.5
connecting to: so
> db.q19329239.runCommand("aggregate", {pipeline: [{$group: {_id: '$user.id', hits: {$sum: 1}}}, {$match: {hits: {$gt: 10}}}], explain: true})
{
    "serverPipeline" : [
        {
            "query" : {

            },
            "projection" : {
                "user.id" : 1,
                "_id" : 0
            },
            "cursor" : {
                "cursor" : "BasicCursor",
                "isMultiKey" : false,
                "n" : 1031,
                "nscannedObjects" : 1031,
                "nscanned" : 1031,
                "nscannedObjectsAllPlans" : 1031,
                "nscannedAllPlans" : 1031,
                "scanAndOrder" : false,
                "indexOnly" : false,
                "nYields" : 0,
                "nChunkSkips" : 0,
                "millis" : 0,
                "indexBounds" : {

                },
                "allPlans" : [
                    {
                        "cursor" : "BasicCursor",
                        "n" : 1031,
                        "nscannedObjects" : 1031,
                        "nscanned" : 1031,
                        "indexBounds" : {

                        }
                    }
                ],
                "server" : "ficrm-rafa.local:27017"
            }
        },
        {
            "$group" : {
                "_id" : "$user.id",
                "hits" : {
                    "$sum" : {
                        "$const" : 1
                    }
                }
            }
        },
        {
            "$match" : {
                "hits" : {
                    "$gt" : 10
                }
            }
        }
    ],
    "ok" : 1
}

Server version.

$ mongo so
MongoDB shell version: 2.4.5
connecting to: so
> db.version()
2.4.5