N1QL query returns Index Scan timed out

simple query 'select `realm`, count from `bucket-1` group by `realm`.
there is a 2i index by_realm on `realm`.
query eventually starts streaming data but before finish reports:
"errors": [

{ "code": 5000, "msg": "Index scan timed out" }

],
"status": "errors",
"metrics":

{ "elapsedTime": "4m19.596748705s", "executionTime": "4m19.596321823s", "resultCount": 670743, "resultSize": 41586066, "errorCount": 1 }

workaround will be to reduce selectivity or use limit, but any query timeout should be user specifiable, and 4m shouldn't be the default!

explain:
cbq> explain select `realm`,count from `bucket-1` group by `realm`;
{
"requestID": "403fc452-7051-4a2f-aa0f-34a769703363",
"signature": "json",
"results": [
{
"#operator": "Sequence",
"~children": [

{ "#operator": "PrimaryScan", "index": "#primary", "keyspace": "bucket-1", "namespace": "default", "using": "gsi" }

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

{ "#operator": "Fetch", "keyspace": "bucket-1", "namespace": "default" }

,

{ "#operator": "InitialGroup", "aggregates": [ "count(*)" ], "group_keys": [ "(`bucket-1`.`realm`)" ] }

,

{ "#operator": "IntermediateGroup", "aggregates": [ "count(*)" ], "group_keys": [ "(`bucket-1`.`realm`)" ] }

]
}
},

{ "#operator": "IntermediateGroup", "aggregates": [ "count(*)" ], "group_keys": [ "(`bucket-1`.`realm`)" ] }

,

{ "#operator": "FinalGroup", "aggregates": [ "count(*)" ], "group_keys": [ "(`bucket-1`.`realm`)" ] }

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

{ "expr": "(`bucket-1`.`realm`)" }

,

{ "expr": "count(*)" }

]
},

{ "#operator": "FinalProject" }

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

{ "elapsedTime": "24.386309ms", "executionTime": "24.215223ms", "resultCount": 1, "resultSize": 3037 }

}

don't know which side times out, but it appears that 2i initiates