Why does Mongo FETCH on count() with $nin? - mongodb

Question

I am trying to understand why Mongo can't use a covered index with my query using $nin, and how to resolve it. My issue is with a compound index, but it happens with a simple index too.
Take a simple document:
{b: "text1"}
And a simple index:
{
"v" : 1,
"key" : {
"b" : 1
},
"name" : "b_1",
"ns" : "mytest"
}
And what I thought was a simple count() query:
db.mytest.count( {b: $nin: [ "foo" ]}, {b:1, _id:0} )
The winningPlan unexpectedly includes a FETCH:
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"b" : 1
},
"indexName" : "b_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"b" : [
"[MinKey, \"foo\")",
"(\"foo\", MaxKey]"
]
}
}
}
}
But with a simple equality condition it uses COUNT_SCAN (as expected):
> db.mytest.count( {b: "bar" }, {b:1, _id:0} )
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "COUNT_SCAN",
"keyPattern" : {
"b" : 1
},
"indexName" : "b_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1
}
},
To make things more interesting, a find() instead of a count() doesn't look at any documents:
> db.mytest.find({b:{ $nin: [ 3 ] }}, {b:1, _id:0})
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"b" : 1,
"_id" : 0
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"b" : 1
},
"indexName" : "b_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"b" : [
"[MinKey, 3.0)",
"(3.0, MaxKey]"
]
}
}
}
Why does Mongo need to FETCH with $nin? It should be able to fulfill this exclusively from the index.

Answer 1

So it appears that this is a bug that was fixed in 3.6. There was definitely an unnecessary FETCH in many COUNT situations.