subqueries are not using index

cbq> CREATE INDEX join_index4 ON default(join_day) ;
{
"request_id": "68e06f32-695e-4f03-bc69-6879fb20d4b1",
"signature": null,
"results": [
],
"status": "success",
"metrics":

{ "elapsedTime": "7.001841s", "executionTime": "7.001374s", "resultCount": 0, "resultSize": 0 }

}

cbq>

{t task_name, (select sum(test_rate) cn from default d use keys ['query-1'] where join_day>2) as names from}

{
"request_id": "6a18431d-62ec-4669-9d81-3de70e243af5",
"signature": "json",
"results": [
{
"#operator": "Sequence",
"~children": [

{ "#operator": "PrimaryScan", "index": "#primary", "keyspace": "default", "namespace": "default" }

,
{
"#operator": "Parallel",
"~child": {
"#operator": "Sequence",
"~children": [

{ "#operator": "Fetch", "keyspace": "default", "namespace": "default" }

,
{
"#operator": "InitialProject",
"result_terms": [

{ "expr": "(`default`.`task_name`)" }

,

{ "as": "names", "expr": "(select 1)" }

]
},

{ "#operator": "FinalProject" }

]
}
}
]
}
],
"status": "success",
"metrics":

{ "elapsedTime": "3.528ms", "executionTime": "3.089ms", "resultCount": 1, "resultSize": 1446 }

}

cbq>