Why mongodb query, 2 conditions slower than 1 condition? - performance

Question

I have 2 queries:
// query 1
{
"site.$id": ObjectId("52d617b5d8c472274f00004f")
}
// query 2
{
"site.$id": ObjectId("52d617b5d8c472274f00004f"),
"category.$id": ObjectId("52d617c0d8c472274f000076")
}
Can someone please explain why the second query took 1s while the first is very fast?
This collection has about 300.000 entries
Here is explain for query 1
db.Product.find({ "site.$id": ObjectId("52d617b5d8c472274f00004f") }).explain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 21,
"nscannedObjects" : 279001,
"nscanned" : 279001,
"nscannedObjectsAllPlans" : 279001,
"nscannedAllPlans" : 279001,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 3,
"nChunkSkips" : 0,
"millis" : 545,
"indexBounds" : {
},
"server" : "ip-172-31-9-78:27017"
}
Here is explain for query 2
db.Product.find({ "site.$id": ObjectId("52d617b5d8c472274f00004f"), "category.$id": ObjectId("52d617c0d8c472274f000076") }).explain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 279002,
"nscanned" : 279002,
"nscannedObjectsAllPlans" : 279002,
"nscannedAllPlans" : 279002,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 107,
"nChunkSkips" : 0,
"millis" : 1852,
"indexBounds" : {
},
"server" : "ip-172-31-9-78:27017"
}

Answer 1

Try to create index for that fields. I think it will work much faster..
db.Product.ensureIndex( { "site.$id": 1, "category.$id": 1 } )
Also you can think about order of fields in your query and respectively in index. This will help http://docs.mongodb.org/manual/tutorial/create-queries-that-ensure-selectivity/