Can MongoDB use an index when checking for existence of a field with $exists operator?

Updated:

Seems $exists queries use index properly now based on these tickets $exists queries should use index & {$exists: false} will not use index

Old Answer:

No, there is no way to tell mongodb to use index for exists query. Indexing is completely related to data. Since $exists is only related to the keys (fields) it cant be used in indexes.

$exists just verifies whether the given key (or field) exists in the document.

$exist will not use index, but you can change your data structure to

photos: [
     {id:123, url: '...', title: '...', ... },
     {id:456, url: '...', title: '...', ... },
     ...
  ]

and then use

db.users.ensureIndex({photos.id:1}) 

to create index for photo id.

It seems I am wrong, in fact, you can force your $exists query to use your index. Let us go on using the above structure, but your photo id is not certainly contained , that is to say some docs will have the key 'id' and some will not. Then you can create sparse index on it:

db.users.ensureIndex({'photos.id': 1}, {'sparse': true})

then query like this:

db.users.find({'photos.id': {$exists: true}}).hint({'photos.id': 1})

you can add explain to see if the query is using index. Here is my result, my collection's mobile key is similar to your photos.id:

> db.test.count()
50000
> db.test.find({'mobile': {$exists: true}}).hint({'mobile': 1}).explain()
{
        "cursor" : "BtreeCursor mobile_1",
        "nscanned" : 49999,
        "nscannedObjects" : 49999,
        "n" : 49999,
        "millis" : 138,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "isMultiKey" : false,
        "indexOnly" : false,
        "indexBounds" : {
                "mobile" : [
                        [
                                {
                                        "$minElement" : 1
                                },
                                {
                                        "$maxElement" : 1
                                }
                        ]
                ]
        }
}

> db.test.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "ns" : "test.test",
                "name" : "_id_"
        },
        {
                "v" : 1,
                "key" : {
                        "mobile" : 1
                },
                "ns" : "test.test",
                "name" : "mobile_1",
                "sparse" : true,
                "background" : true
        }
]

Hope to help!

Since MongoDB 2.0 $exists queries should use an index. Unfortunately this fix has disappeared in the newest version and will be fixed in MongoDB 2.5