force use of index on complex MongoDB query? - mongodb

Question

i have a large collection of "messages" with 'to', 'from', 'type', and 'visible_to' fields that I want to query against with a fairly complex query that pulls only the messages to/from a particular user of a particular set of types that are visible to that user. Here is an actual example:
{
"$and": [
{
"$and": [
{
"$or": [
{
"to": "52f65f592f1d88ebcb00004f"
},
{
"from": "52f65f592f1d88ebcb00004f"
}
]
},
{
"$or": [
{
"type": "command"
},
{
"type": "image"
}
]
}
]
},
{
"$or": [
{
"public": true
},
{
"visible_to": "52f65f592f1d88ebcb00004f"
}
]
}
]
}
With indexes:
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "_id_"
},
{
"v" : 1,
"key" : {
"expires" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "expires_1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"from" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "from_1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"type" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "type_1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"ts" : 1,
"type" : -1
},
"ns" : "n2-mongodb.messages",
"name" : "ts_1_type_-1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"to" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "to_1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"visible_to" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "visible_to_1",
"background" : true,
"safe" : null
},
{
"v" : 1,
"key" : {
"public" : 1,
"visible_to" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "public_1_visible_to_1"
},
{
"v" : 1,
"key" : {
"to" : 1,
"from" : 1
},
"ns" : "n2-mongodb.messages",
"name" : "to_1_from_1"
}
]
And here is the explain(true) output from our MongoDB 2.2.2 instance, which looks like a full scan:
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 35702,
"nscanned" : 35702,
"nscannedObjectsAllPlans" : 35702,
"nscannedAllPlans" : 35702,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 1,
"nChunkSkips" : 0,
"millis" : 85,
"indexBounds" : {
},
"allPlans" : [
{
"cursor" : "BasicCursor",
"n" : 0,
"nscannedObjects" : 35702,
"nscanned" : 35702,
"indexBounds" : {
}
}
],
"server" : "XXXXXXXX"
}
Looking at the explain output, MongoDB is not using any indexes for this - is there a way to get it to use at least the compound index {to: 1, from: 1} to dramatically narrow the search space? Or is there a better way to optimize this query? Or is MongoDB wholly unsuited for a query like this?

Answer 1

To force the MongoDB query optimizer to adopt a specific approach, you can use the $hint operator.
From the docs,
The $hint operator forces the query optimizer to use a specific index to fulfill the query. Specify the index either by the index name or by document.

Answer 2

The query optimizer in MongoDB 2.6 will include support for applying indexes to complex queries.