sparse indexes and null values in mongo

Question

I'm not sure I understand sparse indexes correctly.

I have a sparse unique index on fbId

{
    "ns" : "mydb.users",
    "key" : {
        "fbId" : 1
    },
    "name" : "fbId_1",
    "unique" : true,
    "sparse" : true,
    "background" : false,
    "v" : 0
}

And I was expecting that would allow me to insert records with null as the fbId, but that throws a duplicate key exception. It only allows me to insert if the fbId property is removed completely.

Isn't a sparse index supposed to deal with that?

Answer 1

Sparse indexes do not contain documents that miss indexed field. However, if field exists and has value of null, it will still be indexed. So, if absense of the field and its equality to null look the same for your application and you want to maintain uniqueness of fbId, just don't insert it until you have a value for it.

You need sparse indexes when you have a large number of documents, but only a small portion of them contains some field, and you want to be able to quickly find documents by that field. Creating a normal index would be too expensive, you would just waste precious RAM on indexing documents you're not interested in.