在观看ES文档时做了一些基础测试，在执行以下操作时出现报错

GET /megacorp/employee/_search
{
  "aggs":{
    "all_interests":{
      "terms": {
        "field": "interests"
      }
    }
  }
}

错误如下：

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "Fielddata is disabled on text fields by default. Set fielddata=true on [interests] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead."
      }
    ],
    "type": "search_phase_execution_exception",
    "reason": "all shards failed",
    "phase": "query",
    "grouped": true,
    "failed_shards": [
      {
        "shard": 0,
        "index": "megacorp",
        "node": "3DTOZs6yRsS8kxERdFJoHw",
        "reason": {
          "type": "illegal_argument_exception",
          "reason": "Fielddata is disabled on text fields by default. Set fielddata=true on [interests] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead."
        }
      }
    ],
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "Fielddata is disabled on text fields by default. Set fielddata=true on [interests] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.",
      "caused_by": {
        "type": "illegal_argument_exception",
        "reason": "Fielddata is disabled on text fields by default. Set fielddata=true on [interests] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead."
      }
    }
  },
  "status": 400
}

经过查找验证后发现出现该错误是因为5.x之后，Elasticsearch对排序、聚合所依据的字段用单独的数据结构(fielddata)缓存到内存里了，但是在text字段上默认是禁用的，如果有需要单独开启，这样做的目的是为了节省内存空间。

所以如果需要进行聚合操作，需要单独开启。

执行以下代码：

PUT megacorp/_mapping/employee/
{
  "properties": {
    "interests": { 
      "type":     "text",
      "fielddata": true
    }
  }
}

设置成功后会出现如下结果：

{
  "acknowledged" : true
}

再次执行查询后就出现想要的结果了。
转自：https://blog.csdn.net/abandon_li/article/details/87285887