MongoDB index not covering trivial query - mongodb

Question

I am working on a MongoDB application, and am having trouble with covered queries. After reworking many of my queries to perform better when paginating I found that my previously covered queries were no longer being covered by the index. I tried to distill the working set down as far as possible to isolate the issue but I'm still confused.
First, on a fresh (empty) collection, I inserted the following documents:
devdb> db.test.find()
{ "_id" : ObjectId("53157aa0dd2cab043ab92c14"), "metadata" : { "created_by" : "bcheng" } }
{ "_id" : ObjectId("53157aa6dd2cab043ab92c15"), "metadata" : { "created_by" : "albert" } }
{ "_id" : ObjectId("53157aaadd2cab043ab92c16"), "metadata" : { "created_by" : "zzzzzz" } }
{ "_id" : ObjectId("53157aaedd2cab043ab92c17"), "metadata" : { "created_by" : "thomas" } }
{ "_id" : ObjectId("53157ab9dd2cab043ab92c18"), "metadata" : { "created_by" : "bbbbbb" } }
Then, I created an index for the 'metadata.created_by' field:
devdb> db.test.getIndices()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "devdb.test",
"name" : "_id_"
},
{
"v" : 1,
"key" : {
"metadata.created_by" : 1
},
"ns" : "devdb.test",
"name" : "metadata.created_by_1"
}
]
Now, I tried to lookup a document by the field:
devdb> db.test.find({'metadata.created_by':'bcheng'},{'_id':0,'metadata.created_by':1}).sort({'metadata.created_by':1}).explain()
{
"cursor" : "BtreeCursor metadata.created_by_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"metadata.created_by" : [
[
"bcheng",
"bcheng"
]
]
},
"server" : "localhost:27017"
}
The correct index is being used and no extraneous documents are being scanned. Regardless of the presence of .hint(), limit(), or sort(), indexOnly remains false.
Digging through the documentation, I've seen that covered indices will fail to cover queries on array elements, but that isn't the case here (and isMultiKey shows false).
What am I missing? Are there other reasons for this behavior (eg. insuffient RAM, disk space, etc.)? And if so, how can I best diagnose these issues in the future?

Answer 1

It is currently not supported. See this Jira issue.