Improve querying fields exist in MongoDB

Question

I\'m in progress with estimation of MongoDB for our customers. Per requirements we need associate with some entity ent variable set of name-value pairs.

Answer 1

Simply redesign your schema such that it's an indexable query. Your use case is infact analogous to the first example application given in MongoDB The Definitive Guide.

If you want/need the convenience of result.a just store the keys somewhere indexable.

instead of the existing:

db.ent.insert({a:5775, b:'b1'})

do

db.ent.insert({a:5775, b:'b1', index: ['a', 'b']})

That's then an indexable query:

db.end.find({index: "a"}).explain()
{
    "cursor" : "BtreeCursor index_1",
    "nscanned" : 1,
    "nscannedObjects" : 1,
    "n" : 1,
    "millis" : 0,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : {
        "index" : [
            [
                "a",
                "a"
            ]
        ]
    }
}

or if you're ever likely to query also by value:

db.ent.insert({
    a:5775, 
    b:'b1', 
    index: [
        {name: 'a', value: 5775}, 
        {name: 'b', value: 'b1'}
    ]
})

That's also an indexable query:

db.end.find({"index.name": "a"}).explain()
{
    "cursor" : "BtreeCursor index.name_",
    "nscanned" : 1,
    "nscannedObjects" : 1,
    "n" : 1,
    "millis" : 0,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : {
        "index.name" : [
            [
                "a",
                "a"
            ]
        ]
    }
}