hono: Finding credentials by tenant-id, auth-id and type does not use MongoDb index

The MongoDbDocumentBuilder class uses MongoDb’s $elemMatch operator for filtering credentials by tenant-id, auth-id and type. For this purpose an index on those fields was created. When using MongoDb’s explain it turned out that this index is in fact not used:

db.getCollection('device-credentials').find({
    "tenant-id": "foobar",
    "credentials": {
      "$elemMatch": {
        "auth-id": "spamegg",
        "type": "hashed-password"
      }
    }
  }).explain()
  
# RETURNS
{
    "queryPlanner" : {
        ...
        },
        "winningPlan" : {
            ...
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "tenant-id" : 1,
                    "device-id" : 1
                },
                "indexName" : "tenant-id_1_device-id_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "tenant-id" : [],
                    "device-id" : []
                },
                "isUnique" : true,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "tenant-id" : [ 
                        "[\"foobar\", \"foobar\"]"
                    ],
                    "device-id" : [ 
                        "[MinKey, MaxKey]"
                    ]
                }
            }
        },
        "rejectedPlans" : []
    },
    ...
}

Actually an index on tenant-id and device-id is used and a scan is done for all device ids (see the “[MinKey, MaxKey]” value for “device-id”.

As far as I can see there is no need to the $elemMatch operator for this query as it is only a simple match of two values and not more complicated match resorting to other MongoDb funtions ($gt, $and, $or,…).

If the query is done without the $elemMatch operator the index is used as expected:

db.getCollection('device-credentials').find({
    "tenant-id": "foobar",
    "credentials.auth-id": "spamegg",
    "credentials.type": "hashed-password"
  }).explain()
  
# RETURNS
{
    "queryPlanner" : {
        ...
        },
        "winningPlan" : {
            "stage" : "FETCH",
            "filter" : {
                "credentials.type" : {
                    "$eq" : "hashed-password"
                }
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "tenant-id" : 1,
                    "credentials.auth-id" : 1,
                    "credentials.type" : 1
                },
                "indexName" : "tenant-id_1_credentials.auth-id_1_credentials.type_1",
                "isMultiKey" : true,
                "multiKeyPaths" : {
                    "tenant-id" : [],
                    "credentials.auth-id" : [ 
                        "credentials"
                    ],
                    "credentials.type" : [ 
                        "credentials"
                    ]
                },
                "isUnique" : true,
                "isSparse" : false,
                "isPartial" : true,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "tenant-id" : [ 
                        "[\"foobar\", \"foobar\"]"
                    ],
                    "credentials.auth-id" : [ 
                        "[\"spamegg\", \"spamegg\"]"
                    ],
                    "credentials.type" : [ 
                        "[MinKey, MaxKey]"
                    ]
                }
            }
        },
        "rejectedPlans" : [ 
            ...
        ]
    },
    ...
}

From https://github.com/eclipse/hono/commit/d75a28e7276a055fd5dc72264f8016355cb80223 I understand that the $elemMatch operator was used since the credentials to be returned should not contain sensitive information. Hence MongoDb’s projection operator was used. This did not work with a “plain” filter which is why the $elemMatch operator was introduced.

Since the result set returned by the query should usually be only one credential (consequently there’s no big advantage using the projection so that traffic is reduced), I think that commit could be reverted and the sensitive fields could be removed “in code”.

WDYT?