[BUG] PPL query return null when access valid inner json object

[penghuo]

How can one reproduce the bug?

###
PUT {{baseUrl}}/log00001
Content-Type: application/x-ndjson

{
  "mappings": {
    "properties": {
      "log": {
        "type": "object",
        "enabled": false
      }
    }
  }
}

###
POST {{baseUrl}}/log00001/_bulk
Content-Type: application/x-ndjson

{"index": {}}
{"log": {"a":1, "c": {"d": 2}}}

###
POST {{baseUrl}}/_plugins/_ppl/
Content-Type: application/x-ndjson

{
  "query": "source=log00001 | fields log.c.d"
}

• Results

{
  "schema": [
    {
      "name": "log.c.d",
      "type": "undefined"
    }
  ],
  "datarows": [
    [
      null
    ]
  ],
  "total": 1,
  "size": 1
}

What is the expected behavior?
It should return

{
  "schema": [
    {
      "name": "log.c.d",
      "type": "int"
    }
  ],
  "datarows": [
    [
      2
    ]
  ],
  "total": 1,
  "size": 1
}

[LantaoJin]

It seems only happen when "enabled": false

[LantaoJin]

We cannot get the actual mapping if a field is disabled

PUT disabled
{
  "mappings": {
    "properties": {
      "log": {
        "type": "object",
        "enabled": false
      }
    }
  }
}
PUT enabled
{
  "mappings": {
    "properties": {
      "log": {
        "type": "object"
      }
    }
  }
}

Add data

{"index": {}}
{"log": {"a":1, "c": {"d": 2}}}

GET enabled/_mapping returns

{
  "enabled": {
    "mappings": {
      "properties": {
        "log": {
          "properties": {
            "a": {
              "type": "long"
            },
            "c": {
              "properties": {
                "d": {
                  "type": "long"
                }
              }
            }
          }
        }
      }
    }
  }
}

GET disabled/_mapping returns

{
  "disabled": {
    "mappings": {
      "properties": {
        "log": {
          "type": "object",
          "enabled": false
        }
      }
    }
  }
}

[LantaoJin]

As a result (SQL plugin never know there is a field log.c.d), its type is undefined instead of any since SQL plugin doesn't know there is an internal field log.c.d.
The query source=test | fields log.c.d has no difference with source=test | fields log.c.not_existed_field

{
  "schema": [
    {
      "name": "log.c.not_existed_field",
      "type": "undefined"
    }
  ],
  "datarows": [
    [
      null
    ]
  ],
  "total": 1,
  "size": 1
}

[LantaoJin]

There is a similar case in dynamic mapping

PUT test3
{
  "mappings": {
    "dynamic": false
  }
}

POST test3/_bulk
{"index": {}}
{"a": 1}

Query

POST /_plugins/_ppl/
{
  "query": """
source=test3 | fields a
  """
}

throws "Field [a] not found", because the mapping result doesn't include a

{
  "test3": {
    "mappings": {
      "dynamic": "false"
    }
  }
}

[LantaoJin]

This issue should be resolved by #4984