Question

I created a test MongoDB collection “sampleCollection” with documents which looks like:

 "_id" : ObjectId("510929e041cb2179b41ace1c"),
 "stringField" : "Random string0",
 "longField" : NumberLong(886)

and has index on field “stringField”.
When I execute

db.sampleCollection.find({"stringField":"Random string0"}).explain()

everything is ok:

 "cursor" : "BtreeCursor stringField_1",
 "isMultiKey" : false,
 "n" : 2,
 "nscannedObjects" : 2,
 "nscanned" : 2,
 "nscannedObjectsAllPlans" : 2,
 "nscannedAllPlans" : 2,
 "scanAndOrder" : false,
 "indexOnly" : false,
 "nYields" : 0,
 "nChunkSkips" : 0,
 "millis" : 0,
 "indexBounds" : {
         "stringField" : [
                 [
                         "Random string0",
                         "Random string0"
                 ]
         ]
 }

but

db.sampleCollection.find({$query:{"stringField":"Random string0"}}).explain()

gets me

"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 4,
"nscanned" : 4,
"nscannedObjectsAllPlans" : 4,
"nscannedAllPlans" : 4,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {

}

This is not looks like a problem, but I’m using org.springframework.data.mongodb framework in production and usage of query constructions is an only way to write repositories.
And thus I have a db which completely ignores indexed data.

Is it correct? Or I misunderstood something?

Answer 1

That was funny i cannot decide to say it is a bug or not it is up to you:

There are two available syntax: http://docs.mongodb.org/manual/reference/operator/query/

When you using:

db.collection.find( { age : 25 } )

also will

db.collection.find( { age : 25 } ).explain()
db.collection.find( { age : 25 } ).hint(someindex)

work fine.

When you using your solution (the other syntax):

db.collection.find( { $query: { age : 25 } } )

the output of

db.sampleCollection.find({$query:{"stringField":"Random string0"}}).explain()

Will show like the query not using the index

if you also use .hint for the index it will omit the result. 🙂 (That is i do not really understand)

Fortunately there is another syntax for these operations too: you can use:

db.sampleCollection.find({$query:{"stringField":"Random string0"}, $explain:1})

it will have the right output and showed for me the usage of the index. Also there is similar syntax for $hint.

You can check the documentation here: http://docs.mongodb.org/manual/reference/meta-query-operators/

I found this really interesting so i turned on the profiler:

i made a test collection (queryTst) with around 250k docs each with only _id and an age field in the structure with an index on age.

For this query:

db.queryTst.find({$query:{"age":16},$explain:1})

i got:

{
    "cursor" : "BtreeCursor age_1",
    "isMultiKey" : false,
    "n" : 2,
    "nscannedObjects" : 2,
    "nscanned" : 2,
    "nscannedObjectsAllPlans" : 2,
    "nscannedAllPlans" : 2,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "age" : [
            [
                16,
                16
            ]
        ]
    },
    "allPlans" : [
        {
            "cursor" : "BtreeCursor age_1",
            "n" : 2,
            "nscannedObjects" : 2,
            "nscanned" : 2,
            "indexBounds" : {
                "age" : [
                    [
                        16,
                        16
                    ]
                ]
            }
        }
    ],
    "oldPlan" : {
        "cursor" : "BtreeCursor age_1",
        "indexBounds" : {
            "age" : [
                [
                    16,
                    16
                ]
            ]
        }
    },
    "server" : ""
}

for this:

 db.queryTst.find({$query:{"age":16},$explain:1}).explain()

i got:

"cursor" : "BasicCursor",
    "isMultiKey" : false,
    "n" : 0,
    "nscannedObjects" : 250011,
    "nscanned" : 250011,
    "nscannedObjectsAllPlans" : 250011,
    "nscannedAllPlans" : 250011,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 103,
    "indexBounds" : {

    },

in the profiler log: for the first

{
    "ts" : ISODate("2013-01-30T20:35:40.526Z"),
    "op" : "query",
    "ns" : "test.queryTst",
    "query" : {
        "$query" : {
            "age" : 16
        },
        "$explain" : 1
    },
    "ntoreturn" : 0,
    "ntoskip" : 0,
    "nscanned" : 2,
    "keyUpdates" : 0,
    "numYield" : 0,
    "lockStats" : {
        "timeLockedMicros" : {
            "r" : NumberLong(368),
            "w" : NumberLong(0)
        },
        "timeAcquiringMicros" : {
            "r" : NumberLong(8),
            "w" : NumberLong(5)
        }
    },
    "nreturned" : 1,
    "responseLength" : 567,
    "millis" : 0,
    "client" : "127.0.0.1",
    "user" : ""
}

for the second:

{
    "ts" : ISODate("2013-01-30T20:35:47.715Z"),
    "op" : "query",
    "ns" : "test.queryTst",
    "query" : {
        "query" : {
            "$query" : {
                "age" : 16
            },
            "$explain" : 1
        },
        "$explain" : true
    },
    "ntoreturn" : 0,
    "ntoskip" : 0,
    "nscanned" : 250011,
    "keyUpdates" : 0,
    "numYield" : 0,
    "lockStats" : {
        "timeLockedMicros" : {
            "r" : NumberLong(104092),
            "w" : NumberLong(0)
        },
        "timeAcquiringMicros" : {
            "r" : NumberLong(13),
            "w" : NumberLong(5)
        }
    },
    "nreturned" : 1,
    "responseLength" : 373,
    "millis" : 104,
    "client" : "127.0.0.1",
    "user" : ""
}

which somehow means to me that is the explain() cause the table scan in the mixed syntax.