Multikey sparse index on empty array in MongoDB - mongodb

Question

What behavior is expected in this case? Will mongo treat empty array as null/undefined and include it into sparse index or if array is empty, document won't be indexed?

Answer 1

Empty arrays are not treated the same as null in MongoDB. As you can see in the following code from the Mongo shell, a sparse index finds the empty array as an empty array as opposed to as null.
> c = db.docs
test.docs
> c.insert({a : []})
> c.ensureIndex({a : 1}, {sparse: true})
> c.find({a : []}).count()
1
> c.find({a : null}).count()
0
As with most interesting questions about MongoDB, using explain will provide a wealth of info. For example, you can see that the test actually uses the index and that the boundaries of the index are null and [], demonstrating their unique treatment.
> c.find({a : null}).explain()
{
"cursor" : "BtreeCursor a_1",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"nscannedObjectsAllPlans" : 0,
"nscannedAllPlans" : 0,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[
null,
null
]
]
},
"server" : "new-host.home:27017"
}
> c.find({a : []}).explain()
{
"cursor" : "BtreeCursor a_1 multi",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[
null,
null
],
[
[ ],
[ ]
]
]
},
"server" : "new-host.home:27017"
}