这可以通过摄取管道和script processor

POST _ingest/pipeline/_simulate
{
  "pipeline": {
    "processors": [
      {
        "script": {
          "description": "Summarize the description field",
          "lang": "painless",
          "source": "ctx.preview = ctx.description.substring(0, 20) + '...'"
        }
      }
    ]
  },
  "docs": [
    {
      "_source": {
        "description": "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book."
      }
    }
  ]
}

回复：

{
  "docs" : [
    {
      "doc" : {
        "_source" : {
          "preview" : "Lorem Ipsum is simpl...",
          "description" : "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book."
        }
      }
    }
  ]
}

作为替代方案，您也可以采用固定数量的单词，而不是采用固定数量的字符。很容易改变。