[BUG] The result of reindex depends on whether slices is added or not

[kkewwei]

Describe the bug

1. When we reindex in a empty indices with the command:

PUT nested_12

POST _reindex
{
    "source": {
    "index": "nested_12"
  },
  "dest": {
    "index": "nested_123"
  }
}

The response is as follows:

{
   "took": 71,
   "timed_out": false,
   "total": 0,
   "updated": 0,
   "created": 0,
   "deleted": 0,
   "batches": 0,
   "version_conflicts": 0,
   "noops": 0,
   "retries": {
      "bulk": 0,
      "search": 0
   },
   "throttled_millis": 0,
   "requests_per_second": -1,
   "throttled_until_millis": 0,
   "failures": []
}

2. When the slices is added:

POST _reindex?slices=4
{
    "source": {
    "index": "nested_12"
  },
  "dest": {
    "index": "nested_123"
  }
}

The response is as follows:

{
   "error": {
      "root_cause": [
         {
            "type": "illegal_argument_exception",
            "reason": "field _id not found"
         }
      ],
      "type": "search_phase_execution_exception",
      "reason": "all shards failed",
      "phase": "query",
      "grouped": true,
      "failed_shards": [
         {
            "shard": 0,
            "index": "nested_12",
            "node": "Wj6shio_QcGfnUmKwFb-2w",
            "reason": {
               "type": "illegal_argument_exception",
               "reason": "field _id not found"
            }
         }
      ],

It's caused by

OpenSearch/server/src/main/java/org/opensearch/search/slice/SliceBuilder.java

Line 225 in a2a01f8

throw new IllegalArgumentException("field " + field + " not found");

Related component

Plugins

Expected behavior

The behive should be the same whether slices is added or not

Host/Environment (please complete the following information):

• Version: os2.9

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