MongoDB not uses indexes on some fields when doing distinct - mongodb

Question

I have noticed that MongoDB won't use indexes when querying for a distinct value on a field. I will use it on some fields, but won't on others.
Here's the example:
db.product.createIndex({"_indexed.preventieve_mondzorg-max_bedrag_p_jr": 1});
db.runCommand({distinct: "product", key:"_indexed.preventieve_mondzorg-max_bedrag_p_jr"});
This query will not use an index that is built on that field and will go for the full collection scan. That's what it produces:
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 50,
"numIndexesAfter" : 50,
"note" : "all indexes already exist",
"ok" : 1
}
{
"values" : [
"€ 250,- | 75%",
"Geen dekking",
"...",
],
"stats" : {
"n" : 33660,
"nscanned" : 0,
"nscannedObjects" : 33660,
"timems" : 12531,
"planSummary" : "COLLSCAN"
},
"ok" : 1
}
On the other hand
db.product.createIndex({"free_choice.value": 1});
db.runCommand({distinct: "product", key:"free_choice.value"});
Will the index:
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 50,
"numIndexesAfter" : 50,
"note" : "all indexes already exist",
"ok" : 1
}
{
"values" : [
"gedeeltelijk",
"geen",
"ja"
],
"stats" : {
"n" : 4,
"nscanned" : 4,
"nscannedObjects" : 4,
"timems" : 2,
"planSummary" : "DISTINCT { free_choice.value: 1.0 }"
},
"ok" : 1
}
So... what could be the difference between those two fields?
Is it a bug, or I am doing something wrong?
I am running MongoDB 3.0.6 in a Vagrant box with Ubuntu 14.04.3 LTS

Answer 1

Apparently this is the bug of MongoDB core. Or misbehaviour. MongoDB would not use multikey index for dotted fields for distinct requests.
Here's the Mongo's response:
The distinct optimization uses a special index access stage which
returns the distinct index keys to its parent stage. In the multikey
dotted case, however, the distinct stage would have to check for null
or undefined keys. In the case of null or undefined, it must fetch the
full document in order to disambiguate between literal nulls versus
null by virtue of missing fields. We have decided to hold off unless
we see that users really need this.
If you really want this feature, please vote for it here: SERVER-13298