Why is my mongo query not using index only? - windows

Question

Please, observe:
MongoDB shell version: 2.4.1
connecting to: test
> use dummy
switched to db dummy
> db.invoices.find({'items.nameTags': /^z/}, {_id: 1}).explain()
{
"cursor" : "BtreeCursor items.nameTags_1_created_1_special_1__id_1_items.qty_1_items.total_1 multi",
"isMultiKey" : true,
"n" : 55849,
"nscannedObjects" : 223568,
"nscanned" : 223568,
"nscannedObjectsAllPlans" : 223568,
"nscannedAllPlans" : 223568,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 86,
"nChunkSkips" : 0,
"millis" : 88864,
"indexBounds" : {
"items.nameTags" : [
[
"z",
"{"
],
[
/^z/,
/^z/
]
],
"created" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"special" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"_id" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"items.qty" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"items.total" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "IL-Mark-LT:27017"
}
>
Here is the definition of the index:
> db.system.indexes.find({name : 'items.nameTags_1_created_1_special_1__id_1_items.qty_1_items.total_1'}).pretty()
{
"v" : 1,
"key" : {
"items.nameTags" : 1,
"created" : 1,
"special" : 1,
"_id" : 1,
"items.qty" : 1,
"items.total" : 1
},
"ns" : "dummy.invoices",
"name" : "items.nameTags_1_created_1_special_1__id_1_items.qty_1_items.total_1"
}
>
Finally, here is an example invoice document (with just 2 items):
> db.invoices.findOne({itemCount: 2})
{
"_id" : "85923",
"customer" : "Wgtd Fm 91",
"businessNo" : "314227928",
"billTo_name" : "Wgtd Fm 91",
"billTo_addressLine1" : "3839 Ross Street",
"billTo_addressLine2" : "Kingston, ON",
"billTo_postalCode" : "K7L 4V4",
"purchaseOrderNo" : "boi",
"terms" : "COD",
"shipDate" : "2013-07-10",
"shipVia" : "Moses Transportation Inc.",
"rep" : "Snowhite",
"items" : [
{
"qty" : 4,
"name" : "CA 7789",
"desc" : "3 pc. Coffee Table set (Silver)",
"price" : 222.3,
"total" : 889.2,
"nameTags" : [
"ca 7789",
"a 7789",
" 7789",
"7789",
"789",
"89",
"9"
],
"descTags" : [
"3",
"pc",
"c",
"coffee",
"offee",
"ffee",
"fee",
"ee",
"e",
"table",
"able",
"ble",
"le",
"e",
"set",
"et",
"t",
"silver",
"ilver",
"lver",
"ver",
"er",
"r"
]
},
{
"qty" : 4,
"name" : "QP 8681",
"desc" : "Ottoman Bed",
"price" : 1179.1,
"total" : 4716.4,
"nameTags" : [
"qp 8681",
"p 8681",
" 8681",
"8681",
"681",
"81",
"1"
],
"descTags" : [
"ottoman",
"ttoman",
"toman",
"oman",
"man",
"an",
"n",
"bed",
"ed",
"d"
]
}
],
"itemCount" : 2,
"discount" : "10%",
"delivery" : 250,
"hstPercents" : 13,
"subTotal" : 5605.6,
"totalBeforeHST" : 5295.04,
"total" : 5983.4,
"totalDiscount" : 560.56,
"hst" : 688.36,
"modified" : "2012-10-08",
"created" : "2014-06-25",
"version" : 0
}
>
My problem is that mongodb does not use index only according to the aforementioned explain() output. Why? After all I only request the _id field, which is part of the index.
In general, I feel that I am doing something very wrong. My invoices collection has 65,000 invoices with the total of 3,291,092 items. It took almost 89 seconds to explain() the query.
What am I doing wrong?

Answer 1

You are using arrays and subdocuments. Covered Indexes dont work with either of these.
From the mongo docs:
An index cannot cover a query if:
any of the indexed fields in any of the documents in the collection includes an array. If an indexed field is an array, the index becomes a multi-key index index and cannot support a covered query.
any of the indexed fields are fields in subdocuments. To index fields in subdocuments, use dot notation. For example, consider a collection users with documents of the following form:
http://docs.mongodb.org/manual/tutorial/create-indexes-to-support-queries/