> db.index.find({val: {$not: {$gte: null}}})

> db.non.find({val: {$not: {$gte: null}}})

{ "_id" : ObjectId("5f16f0bb1fe2fb0ca7b13d12"), "val" : [ ] }

> db.index.find()

{ "_id" : ObjectId("5f16f0da1fe2fb0ca7b13d13"), "val" : [ ] }

{ "_id" : ObjectId("5f16ff661fe2fb0ca7b13d14"), "val" : null }

> db.non.find()

{ "_id" : ObjectId("5f16f0bb1fe2fb0ca7b13d12"), "val" : [ ] }

{ "_id" : ObjectId("5f16ff6d1fe2fb0ca7b13d15"), "val" : null }

> db.index.getIndexes()

[

        {

                "v" : 2,

                "key" : {

                        "_id" : 1

                },

                "name" : "_id_"

        },

        {

                "v" : 2,

                "key" : {

                        "val" : 1

                },

                "name" : "val_1"

        }

]

> db.non.getIndexes()

[ { "v" : 2, "key" : { "_id" : 1 }, "name" : "_id_" } ]