Query slowly even index works on meta().id

n1ql
#1

Hi, I am using Couchbase Server 4.1.0-5005 Enterprise Edition (build-5005).
and I has created a index by:

CREATE INDEX idx_pk_prefix on `db`(substr(meta().id,0,6));

when I execute the following N1QL:

select * from db where substr(meta().id,0,6) != "_sync:" limit 1000;

the result shows that:

Execution:21.07s
Result Count: 1000

but when I execute:

explain select * from db where substr(meta().id,0,6) != "_sync:";

It shows index works:

[
  {
    "#operator": "Sequence",
    "~children": [
      {
        "#operator": "UnionScan",
        "scans": [
          {
            "#operator": "IndexScan",
            "index": "idx_pk_prefix",
            "keyspace": "db",
            "namespace": "default",
            "spans": [
              {
                "Range": {
                  "High": [
                    "\"_sync:\""
                  ],
                  "Inclusion": 0
                }
              },
              {
                "Range": {
                  "Inclusion": 0,
                  "Low": [
                    "\"_sync:\""
                  ]
                }
              }
            ],
            "using": "gsi"
          }
        ]
      },
      {
        "#operator": "Parallel",
        "~child": {
          "#operator": "Sequence",
          "~children": [
            {
              "#operator": "Fetch",
              "keyspace": "db",
              "namespace": "default"
            },
            {
              "#operator": "Filter",
              "condition": "(not (substr((meta(`db`).`id`), 0, 6) = \"_sync:\"))"
            },
            {
              "#operator": "InitialProject",
              "result_terms": [
                {
                  "expr": "self",
                  "star": true
                }
              ]
            },
            {
              "#operator": "FinalProject"
            }
          ]
        }
      }
    ]
  }
]

PS: I have total 9660 docs in my db.
In fact, when I am not create index, the query is much faster(Execution:16.24s)
I want to know why?

#2

I even create a index by:

CREATE INDEX idx_pk_ids on `db`(meta().id) where substr(meta().id,0,6) != "_sync:";

but it seams index don’t work.