indexing - mongodb index range find -

if have these docs:

{"name": 1, "score": 2} {"name": 1, "score": 4} {"name": 2, "score": 2} {"name": 2, "score": 4}

if ensure index:

db.test.ensureindex({"name":1, "score":1})

then try find():

db.test.find({"score": 4})

i use explain(), , found query can not use index , scan 4 docs.

i wonder why scans docs?

you know, if enum "name" 's value (1 , 2):

db.test.find({"$or":["name":1, "name":2], "score":4})

it can use index , scanned 2 docs.

why mongodb can not thing me?

the reason index not being used because $or exclusion operator, , means more or less scans in order determine not match.

what want use $in. reduce match only values contained in set.

db.test.find({"$in":["name":1, "name":2], "score":4})

as inclusive match can applied index without doing full scan.

KBPS