composite indexes: expected index is not used

how user can know what index will be used?
I have items like
{
"tasks_points":

{ "task1": 1, "task2": 1 }

,
"name": "employee-20",
"mutated": 0,
"skills": [
"skill2010",
"skill2011"
],
"join_day": 20,
"join_mo": 5,
"email": "20-mail@couchbase.com",
"test_rate": 5.5,
"join_yr": 2010,
"_id": "query-test003d2ed-0",
"VMs": [

{ "RAM": 5, "os": "ubuntu", "name": "vm_5", "memory": 5 }

,

{ "RAM": 5, "os": "windows", "name": "vm_6", "memory": 5 }

],
"job_title": "Sales"
}

and 2 indexes like:
cbq> select * from system:indexes where "`join_yr`" IN index_key;
{
"requestID": "9283861d-b0e9-4e87-9ab8-e3b705911161",
"signature":

{ "*": "*" }

,
"results": [
{
"indexes":

{ "datastore_id": "http://127.0.0.1:8091", "id": "c0522d4194ddcd93", "index_key": [ "`join_yr`" ], "keyspace_id": "default", "name": "ind11", "namespace_id": "default", "state": "online", "using": "gsi" }

},
{
"indexes":

{ "datastore_id": "http://127.0.0.1:8091", "id": "f87e6abe173f5de", "index_key": [ "`join_yr`", "`VMs`" ], "keyspace_id": "default", "name": "ind19", "namespace_id": "default", "state": "online", "using": "gsi" }

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

{ "elapsedTime": "12.662550022s", "executionTime": "12.662423452s", "resultCount": 2, "resultSize": 858 }

}

cbq>

the query "select join_yr, VMs from default where join_yr>2 and VMs is not null" has to verify join_yr and VMs.
I would expect that ind19 will be used, because it has both fields. But now ind11 is used which has only one of the fields.
Index for second field also exists.
{
"indexes":

{ "datastore_id": "http://127.0.0.1:8091", "id": "d62959e45d1f1542", "index_key": [ "`VMs`" ], "keyspace_id": "default", "name": "ind13", "namespace_id": "default", "state": "online", "using": "gsi" }

},