MongoDB indexes for $elemMatch - mongodb

Question

The indexes help page at http://www.mongodb.org/display/DOCS/Indexes doesn't mention $elemMatch and since it says to add an index on my 2M+ object collection I thought I'd ask this:
I am doing a query like:
{ lc: "eng", group: "xyz", indices: { $elemMatch: { text: "as", pos: { $gt: 1 } } } }
If I add an index
{lc:1, group:1, indices.text:1, indices.pos:1}
will this query with the $elemMatch component be able to be fully run through the index?

Answer 1

Based on your query, I imagine that your documents look something like this:
{
"_id" : 1,
"lc" : "eng",
"group" : "xyz",
"indices" : [
{
"text" : "as",
"pos" : 2
},
{
"text" : "text",
"pos" : 4
}
]
}
I created a test collection with documents of this format, created the index, and ran the query that you posted with the .explain() option.
The index is being used as expected:
> db.test.ensureIndex({"lc":1, "group":1, "indices.text":1, "indices.pos":1})
> db.test.find({ lc: "eng", group: "xyz", indices: { $elemMatch: { text: "as", pos: { $gt: 1 } } } }).explain()
{
"cursor" : "BtreeCursor lc_1_group_1_indices.text_1_indices.pos_1",
"isMultiKey" : true,
"n" : NumberLong(1),
"nscannedObjects" : NumberLong(1),
"nscanned" : NumberLong(1),
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : NumberLong(0),
"millis" : 0,
"indexBounds" : {
"lc" : [
[
"eng",
"eng"
]
],
"group" : [
[
"xyz",
"xyz"
]
],
"indices.text" : [
[
"as",
"as"
]
],
"indices.pos" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "Marcs-MacBook-Pro.local:27017"
}
The documentation on the .explain() feature may be found here: http://www.mongodb.org/display/DOCS/Explain
.explain() may be used to display information about a query, including which (if any) index is used.