Mongodb db.collection.distinct() on aws documentdb doesn't use index - amazon-web-services

Question

Transitioning to new AWS documentDB service. Currently, on Mongo 3.2. When I run db.collection.distinct("FIELD_NAME") it returns the results really quickly. I did a database dump to AWS document DB (Mongo 3.6 compatible) and this simple query just gets stuck.
Here's my .explain() and the indexes on the working instance versus AWS documentdb:
Explain function on working instance:
> db.collection.explain().distinct("FIELD_NAME")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db.collection",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [ ]
},
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"_id" : 0,
"FIELD_NAME" : 1
},
"inputStage" : {
"stage" : "DISTINCT_SCAN",
"keyPattern" : {
"FIELD_NAME" : 1
},
"indexName" : "FIELD_INDEX_NAME",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"FIELD_NAME" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
Explain on AWS documentdb, not working:
rs0:PRIMARY> db.collection.explain().distinct("FIELD_NAME")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db.collection",
"winningPlan" : {
"stage" : "AGGREGATE",
"inputStage" : {
"stage" : "HASH_AGGREGATE",
"inputStage" : {
"stage" : "COLLSCAN"
}
}
}
},
}
Index on both of these instances:
{
"v" : 1,
"key" : {
"FIELD_NAME" : 1
},
"name" : "FIELD_INDEX_NAME",
"ns" : "db.collection"
}
Also, this database has a couple million documents but there are only about 20 distinct values for that "FIELD_NAME". Any help would be appreciated.
I tried it with .hint("index_name") and that didn't work. I tried clearing plan cache but I get Feature not supported: planCacheClear

Answer 1

COLLSCAN and IXSCAN don't have too much difference in this case, both need to scan all the documents or index entries.