mongodb not using indexes when sorting?

Question

i have a collection with these indexes:

> db.message.getIndexKeys()
[
    {
        "_id" : 1
    },
    {
        "msgid" : 1
    },
    {
        "keywords" : 1,
        "msgid" : 1
    }
]

and a query like

db.message.find({'keywords': {'$all': ['apple', 'banana']}}).limit(30).explain()

works fine with index

{
    "cursor" : "BtreeCursor keywords_1_msgid_1",    
    "nscanned" : 96,
    "nscannedObjects" : 96,
    ...
}

but when sorting with msgid:

db.message.find({'keywords': {'$all': ['apple', 'banana']}})
    .sort({msgid:-1})
    .limit(30).explain()

mongodb do not use indexes any more:

{
"cursor" : "BtreeCursor msgid_1 reverse",
"nscanned" : 1784455,
"nscannedObjects" : 1784455,
...
}

any solutions?

Answer 1

Mongo actually is using an index (which you can tell by seeing BtreeCursor in the explain), just not the compound one.

It's important to keep in mind that direction matters when you have a compound index.

Try: db.ensureIndex({ keywords: 1, msg_id: -1 })

Mongo chooses to use msg_id index in reverse in your example because its faster to retrieve the results in sorted order and then match in O(n) time than to match the results and then sort in O(nlogn) time.