MongoDB $or query with index - mongodb

Question

I need make a query with indice at the mongodb, I will show below a minimal case example of my real case.
I have following collection with flowing data:
devsrv(mongod-3.0.4) test> db.teste.find()
{
"_id": ObjectId("57b324c341aaa4b930ef3b92"),
"a": 1,
"b": 1
}
{
"_id": ObjectId("57b324c941aaa4b930ef3b93"),
"a": 1,
"b": 2
}
{
"_id": ObjectId("57b324cd41aaa4b930ef3b94"),
"a": 1,
"b": 3
}
{
"_id": ObjectId("57b324d141aaa4b930ef3b95"),
"a": 1,
"b": 4
}
{
"_id": ObjectId("57b324d541aaa4b930ef3b96"),
"a": 1,
"b": 5
}
{
"_id": ObjectId("57b324da41aaa4b930ef3b97"),
"a": 1,
"b": 6
}
{
"_id": ObjectId("57b324df41aaa4b930ef3b98"),
"a": 1,
"b": 7
}
{
"_id": ObjectId("57b324e441aaa4b930ef3b99"),
"a": 1,
"b": 8
}
{
"_id": ObjectId("57b324f341aaa4b930ef3b9a"),
"a": 1,
"b": ""
}
{
"_id": ObjectId("57b324f641aaa4b930ef3b9b"),
"a": 1,
"b": " "
}
{
"_id": ObjectId("57b324fc41aaa4b930ef3b9c"),
"a": 1,
"b": null
}
{
"_id": ObjectId("57b3250341aaa4b930ef3b9d"),
"a": 1
}
{
"_id": ObjectId("57b46ace41aaa4b930ef3b9e"),
"a": 2
}
And I have the following indexes:
devsrv(mongod-3.0.4) test> db.teste.getIndexes()
[
{
"v": 1,
"key": {
"_id": 1
},
"name": "_id_",
"ns": "test.teste"
},
{
"v": 1,
"key": {
"a": 1,
"b": 1
},
"name": "a_1_b_1",
"ns": "test.teste"
},
{
"v": 1,
"key": {
"b": 1
},
"name": "b_1",
"ns": "test.teste"
}
]
And I need make a query equal this:
devsrv(mongod-3.0.4) test> db.teste.find({$or:[{"b":null},{"b":""},{"b":" "},{"b":{$lt:3}}],"a":1}).explain("executionStats")
{
"queryPlanner": {
"plannerVersion": 1,
"namespace": "test.teste",
"indexFilterSet": false,
"parsedQuery": {
"$and": [
{
"$or": [
{
"b": {
"$eq": null
}
},
{
"b": {
"$eq": ""
}
},
{
"b": {
"$eq": " "
}
},
{
"b": {
"$lt": 3
}
}
]
},
{
"a": {
"$eq": 1
}
}
]
},
"winningPlan": {
"stage": "FETCH",
"filter": {
"a": {
"$eq": 1
}
},
"inputStage": {
"stage": "FETCH",
"filter": {
"$or": [
{
"b": {
"$eq": null
}
},
{
"b": {
"$eq": ""
}
},
{
"b": {
"$eq": " "
}
},
{
"b": {
"$lt": 3
}
}
]
},
"inputStage": {
"stage": "IXSCAN",
"keyPattern": {
"b": 1
},
"indexName": "b_1",
"isMultiKey": false,
"direction": "forward",
"indexBounds": {
"b": [
"[null, null]",
"[-inf.0, 3.0)",
"[\"\", \"\"]",
"[\" \", \" \"]"
]
}
}
}
},
"rejectedPlans": [
{
"stage": "FETCH",
"filter": {
"$or": [
{
"b": {
"$eq": null
}
},
{
"b": {
"$eq": ""
}
},
{
"b": {
"$eq": " "
}
},
{
"b": {
"$lt": 3
}
}
]
},
"inputStage": {
"stage": "IXSCAN",
"keyPattern": {
"a": 1,
"b": 1
},
"indexName": "a_1_b_1",
"isMultiKey": false,
"direction": "forward",
"indexBounds": {
"a": [
"[1.0, 1.0]"
],
"b": [
"[MinKey, MaxKey]"
]
}
}
}
]
},
"executionStats": {
"executionSuccess": true,
"nReturned": 6,
"executionTimeMillis": 0,
"totalKeysExamined": 8,
"totalDocsExamined": 14,
"executionStages": {
"stage": "FETCH",
"filter": {
"a": {
"$eq": 1
}
},
"nReturned": 6,
"executionTimeMillisEstimate": 0,
"works": 10,
"advanced": 6,
"needTime": 2,
"needFetch": 0,
"saveState": 0,
"restoreState": 0,
"isEOF": 1,
"invalidates": 0,
"docsExamined": 7,
"alreadyHasObj": 7,
"inputStage": {
"stage": "FETCH",
"filter": {
"$or": [
{
"b": {
"$eq": null
}
},
{
"b": {
"$eq": ""
}
},
{
"b": {
"$eq": " "
}
},
{
"b": {
"$lt": 3
}
}
]
},
"nReturned": 7,
"executionTimeMillisEstimate": 0,
"works": 8,
"advanced": 7,
"needTime": 1,
"needFetch": 0,
"saveState": 0,
"restoreState": 0,
"isEOF": 1,
"invalidates": 0,
"docsExamined": 7,
"alreadyHasObj": 0,
"inputStage": {
"stage": "IXSCAN",
"nReturned": 7,
"executionTimeMillisEstimate": 0,
"works": 8,
"advanced": 7,
"needTime": 1,
"needFetch": 0,
"saveState": 0,
"restoreState": 0,
"isEOF": 1,
"invalidates": 0,
"keyPattern": {
"b": 1
},
"indexName": "b_1",
"isMultiKey": false,
"direction": "forward",
"indexBounds": {
"b": [
"[null, null]",
"[-inf.0, 3.0)",
"[\"\", \"\"]",
"[\" \", \" \"]"
]
},
"keysExamined": 8,
"dupsTested": 0,
"dupsDropped": 0,
"seenInvalidated": 0,
"matchTested": 0
}
}
}
},
"serverInfo": {
"host": "devsrv",
"port": 27017,
"version": "3.0.4",
"gitVersion": "0481c958daeb2969800511e7475dc66986fa9ed5"
},
"ok": 1
}
But MongoDB isn't using the two indexes together.

Answer 1

Each $or term is effectively a separate query, so it helps to structure your query so that each term aligns with the index you're hoping to use. In this case that means moving the a: 1 part inside of each $or term:
db.teste.find({
$or:[
{a: 1, b: null},
{a: 1, b: ""},
{a: 1, b: " "},
{a: 1, b: {$lt: 3}}
]}).explain('executionStats')
The explain output shows that the a_1_b_1 is used for this query.
But you can simplify this a bit more by using $in to combine the first three terms into one:
db.teste.find({
$or:[
{a: 1, b: {$in: [null, "", " "]}},
{a: 1, b: {$lt: 3}}
]}).explain('executionStats')
This is also able to use the a_1_b_1 index.

Answer 2

The code is
db.teste.explain("executionStats").find({a: 1,
$or:[{b: null},
{b: ""},
{b: " "},
{b: {$lt:3}}]
}).hint({a: 1, b: 1})
Be careful with the hint command as the query optimizer chooses the most efficient query by measuring actual performance of the query with every suitable index.