Cast not used in the where condition

[myamor163]

select cast(offset as int) from file1 #The query is successful

select * from file1 cast(offset as int) >20 # Error:

{"error":{"root_cause":[{"type":"null_pointer_exception","reason":null}],"type":"null_pointer_exception","reason":null},"status":500}

How to solve this problem？

[shi-yuan]

select * from file1 where cast(offset as int) > 20

{
  "from": 0,
  "size": 1000,
  "query": {
    "bool": {
      "filter": [
        {
          "bool": {
            "must": [
              {
                "script": {
                  "script": {
                    "source": "def field_2099360277 = Double.parseDouble(doc['offset'].value.toString()).intValue();((Comparable)field_2099360277).compareTo(20) > 0",
                    "lang": "painless"
                  },
                  "boost": 1
                }
              }
            ],
            "adjust_pure_negative": true,
            "boost": 1
          }
        }
      ],
      "adjust_pure_negative": true,
      "boost": 1
    }
  }
}