Multi-key Indexes and Arrays: MongoDB

We have learnt the basics of multi-key indexes in MongoDB. Lets look at an example to demonstrate the multi-key indexing on arrays.

foo: database name
name: collection name

Insert a document

MongoDB shell version: 2.6.1
connecting to: test

> use foo
switched to db foo

> db.name.insert({a: 1, b: 2, c: 3});
WriteResult({ "nInserted" : 1 })

Here we insert {a: 1, b: 2, c: 3} into “name” collection.

Multi-key Indexes and Arrays: MongoDB

Basic Cursor

> db.name.find({a: 1, b: 2})
{ "_id" : ObjectId("53d8982b79142c385cddc607"), "a" : 1, "b" : 2, "c" : 3 }

> db.name.find({a: 1, b: 2}).explain()
{
        "cursor" : "BasicCursor",
        "isMultiKey" : false,
        "n" : 1,
        "nscannedObjects" : 1,
        "nscanned" : 1,
        "nscannedObjectsAllPlans" : 1,
        "nscannedAllPlans" : 1,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "server" : "Satish-PC:27017",
        "filterSet" : false
}

We find() the document using fields “a” and “b” and the query/command returns a basic cursor, as we do not have indexing on them.

Related Read: index creation: MongoDB

Lets create index on a and b

> db.name.ensureIndex({a: 1, b: 1});
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}

Previous there was only 1 index i.e., on “_id” Now there are 2 indexes – “_id” and “{a: 1, b: 1}”

Btree Cursor with multi-key as false

> db.name.find({a: 1, b: 2}).explain()
{
        "cursor" : "BtreeCursor a_1_b_1",
        "isMultiKey" : false,
        "n" : 1,
        "nscannedObjects" : 1,
        "nscanned" : 1,
        "nscannedObjectsAllPlans" : 1,
        "nscannedAllPlans" : 1,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "a" : [
                        [
                                1,
                                1
                        ]
                ],
                "b" : [
                        [
                                2,
                                2
                        ]
                ]
        },
        "server" : "Satish-PC:27017",
        "filterSet" : false
}

After creating the index on “a” and “b”, chain explain() method on the same command, and it shows you that, now it returns a Btree Cursor.

Lets insert another document

> db.name.insert({a: [0, 1, 2], b: 2, c: 3});
WriteResult({ "nInserted" : 1 })

Lets insert an array as value to field “a” and scalar values to “b” and “c”.

Btree Cursor with Multi-key true

> db.name.find({a: 1, b: 2})
{ "_id" : ObjectId("53d8982b79142c385cddc607"), 
  "a" : 1, "b" : 2, "c" : 3 }
{ "_id" : ObjectId("53d8986f79142c385cddc608"), 
  "a" : [ 0, 1, 2 ], "b" : 2, "c": 3 }

> db.name.find({a: 1, b: 2}).explain()
{
        "cursor" : "BtreeCursor a_1_b_1",
        "isMultiKey" : true,
        "n" : 2,
        "nscannedObjects" : 2,
        "nscanned" : 2,
        "nscannedObjectsAllPlans" : 2,
        "nscannedAllPlans" : 2,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "a" : [
                        [
                                1,
                                1
                        ]
                ],
                "b" : [
                        [
                                2,
                                2
                        ]
                ]
        },
        "server" : "Satish-PC:27017",
        "filterSet" : false
}

Now append explain() method to our command, it shows us that it returns a Btree Cursor and multi-key as true. MongoDB engine need to match every element of the array present in field “a” with the scalar value of field “b”. Hence it uses Multi-Key indexing.

Multi-Key Condition in MongoDB

> db.name.insert({a: [0, 1, 2], b: [3, 4], c: 3});
WriteResult({
        "nInserted" : 0,
        "writeError" : {
                "code" : 10088,
                "errmsg" : "insertDocument :: caused by :: 10088 cannot 
                            index parallel arrays [b] [a]"
        }
})

It’s difficult to match every combination of the array elements present inside both “a” and “b” fields. If both keys/indexes has its value as an array, then it gets complicated. Thus, mongoDB doesn’t allow both keys to be arrays. Either one of them must be a scalar value.