Delete_by_query task API returns some slices as null

[mats990 (Matija Bruncic)]

Hi,

I'm using delete_by_query in ES version 5.6.4 and I'm using it with slices. Example of my request:

POST index-pattern/doc/_delete_by_query?slices=20&wait_for_completion=false&scroll_size=3000

After that I'm using task API to get info :

GET _tasks/VzPPBNs6S4uUQLl1b8LVUw:1406062032

which returns unusual reponse:

{
"completed": false,
"task": {
"node": "VzPPBNs6S4uUQLl1b8LVUw",
"id": 1406062032,
"type": "transport",
"action": "indices:data/write/delete/byquery",
"status": {
  "total": 12345,
  "updated": 0,
  "created": 0,
  "deleted": 1234,
  "batches": 59,
  "version_conflicts": 9663,
  "noops": 0,
  "retries": {
    "bulk": 0,
    "search": 0
  },
  "throttled_millis": 31799,
  "requests_per_second": 30000,
  "throttled_until_millis": 0,
  "slices": [
    {
      "slice_id": 0,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 12345,
      "batches": 9,
      "version_conflicts": 8,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 4799,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    null,
    null,
    null,
    {
      "slice_id": 4,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 12345,
      "batches": 10,
      "version_conflicts": 1416,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 5399,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    null,
    {
      "slice_id": 6,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 0,
      "batches": 1,
      "version_conflicts": 3000,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 0,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    {
      "slice_id": 7,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 12345,
      "batches": 37,
      "version_conflicts": 743,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 21599,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    null,
    null,
    null,
    null,
    null,
    null,
    null,
    {
      "slice_id": 15,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 12345,
      "batches": 1,
      "version_conflicts": 1496,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 0,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    null,
    null,
    {
      "slice_id": 18,
      "total": 12345,
      "updated": 0,
      "created": 0,
      "deleted": 0,
      "batches": 1,
      "version_conflicts": 3000,
      "noops": 0,
      "retries": {
        "bulk": 0,
        "search": 0
      },
      "throttled_millis": 0,
      "requests_per_second": 5000,
      "throttled_until_millis": 0
    },
    null
  ]
},
"description": "delete-by-query [index-pattern",
"start_time_in_millis": 12345,
"running_time_in_nanos": 12345,
"cancellable": true
}
}

My question is: why are there slices that are null?