Mongo Index not being used - mongodb

Question

I created an index around several items for a particular query I am doing:
{
"v" : 1,
"key" : {
"MODIFIED" : -1,
"state" : 1,
"fail" : 1,
"generated" : 1
},
"ns" : "foo.bar",
"name" : "MODIFIED_-1_state_1_fail_1_generated"
}
However when I execute my query, it doesn't apear to be using my index. Could you please provide some insite into what I'm doing wrong?
Thank you!
db.foo.find( {
"$or": [
{
"MODIFIED": {
"$gt": {
"sec": 1321419600,
"usec": 0
}
}
},
{
"$or": [
{"state": "ca"},
{"state": "ok"}
]
}
],
"$and": [
{"fail": {"$ne": 1}},
{"generated": {"$exists": false}}
]
}).explain();
{
"cursor" : "BasicCursor",
"nscanned" : 464215,
"nscannedObjects" : 464215,
"n" : 0,
"millis" : 7549,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
}
}

Answer 1

There's a good reason your index cannot be used for your query and I also think there are some issues with the query itself. The reason it's not hitting an index is because of the nested $or operator by the way but I think your actual problem is a lack of understanding on all the operators available to you in MongoDB :
First of all, your nested $or to check if the state is either "ca" or "ok" is not necessary and ( since it's the main reason you're not hitting your index) can be replaced with state:{$in:["ca", "ok"]} which does the exact same thing. Now your query is :
db.foo.find( {
"$or": [
{
"MODIFIED": {
"$gt": {
"sec": 1321419600,
"usec": 0
}
}
},
{
state:{$in:["ca", "ok"]}
}
],
"$and": [
{"fail": {"$ne": 1}},
{"generated": {"$exists": false}}
]
}).explain();
And it will hit your index. Your second issue is that a top-level $and clause is not necessary. Note that AND(OR(A, B), AND(C, D)) = AND(OR(A, B), C, D). This query does the same :
db.foo.find( {
"$or": [
{
"MODIFIED": {
"$gt": {
"sec": 1321419600,
"usec": 0
}
}
},
{
state:{$in:["ca", "ok"]}
}
],
"fail": {"$ne": 1},
"generated": {"$exists": false}
}).explain();
Which still hits the index :
{
"clauses" : [
{
"cursor" : "BtreeCursor MODIFIED_-1_state_1_fail_1_generated_1 multi",
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 1,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"MODIFIED" : [
[
{
"$maxElement" : 1
},
{
"sec" : 1321419600,
"usec" : 0
}
]
],
"state" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"fail" : [
[
{
"$minElement" : 1
},
1
],
[
1,
{
"$maxElement" : 1
}
]
],
"generated" : [
[
null,
null
]
]
}
},
{
"cursor" : "BasicCursor",
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 1,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
}
}
],
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 1
}
Hope that helps! By the way it's slightly more conventional to start the first key in your compound index with order 1 and the second with -1. Note that the -1 is only used to determine the direction relative to the previous field.