Question

i have a structure of document in mongodb as below

{
  "_id": "some_id",
    "key1": "value1",
    "key2": "value2",
    "key3": [
             {"subkey1":"subval1", "subkey2":"subval2",   "subkey3":"subval3"},
             {"subkey1":"subval11", "subkey2":"subval21", "subkey3":"subval31"},
             {"subkey1":"subval12", "subkey2": "subval22", "subkey3":"subval32"}
            ],
    "key4": "value4"
}

and i want to put index on “subkey1”. how to do it???

or is it not possible(in this case is it work that much efficiently with the index on “key3”)???

Answer 1

just use ensureIndex to add an index to the key you want

db.foos.ensureIndex({"key3.subkey1": 1})

as indicated in the docs:

When a document’s stored value for a index key field is an array, MongoDB indexes each element of the array

adding an index to key3 alone, would not be used if you query using “key3.subkey1”.