This might be a shaded line between a bug and a feature request. Or maybe a documentation request because it's not clear why it doesn't work.

 

We have a document as follows:

```

{   _id: ObjectId(...),   account: ObjectId(...),   names: ["name1", "name2"],   ... }

```

 

We make a compound + multikey + partial filter expression index as follows:

```

{ account: 1, names: 1}

, {partialFilterExpression: {"names":

{"$exists": 1}

}}

```

This works and searches on account + names use the index.

But names is in a minority of documents, so if we have a document with an empty array, it indexes the document, bloating the index size. (we should probably make them `null` indeed).

So we try this clever index:

```

{ account: 1, names: 1}

, {partialFilterExpression: {"names.0":

{"$exists": 1}

}}

```

Now the index is created only on arrays where `names.0` exists. However when we query on `account` and `names` it doesn't use the index. Either it doesn't use this because that's just not how multikey indexes work - or it doesn't work because the query planner doesn't see `names.0` as a match for the query so it rejects the idea of using the index.