Elastic Queries with aliases const keyword and non-const keyword

queries_const_keyword.json

# Mapping with a "constant_keyword" for "data_stream.dataset"
# Mapping with an alias from "event.dataset" -> "data_stream.dataset"
DELETE const-keyword-frank-delme-1
PUT const-keyword-frank-delme-1
{
  "mappings": {
    "dynamic":"false",
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "message": {
        "type": "text"
      },
      "data_stream": {
        "properties": {
          "dataset": {
            "type": "constant_keyword",
            "value": "endpoint"
          }
        }
      },
      "event": {
        "properties": {
          "dataset": {
            "type": "alias",
            "path": "data_stream.dataset"
          }
        }
      }
    }
  }
}

# Non data_stream mapping with "event.dataset" as type "keyword" and no data_stream
DELETE keyword-frank-delme-1
PUT keyword-frank-delme-1
{
  "mappings": {
    "dynamic":"false",
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "message": {
        "type": "text"
      },
      "event": {
        "properties": {
          "dataset": {
            "type": "keyword",
            "ignore_above" : 1024
          }
        }
      }
    }
  }
}

# post example document to keyword
DELETE keyword-frank-delme-1/_doc/1
POST keyword-frank-delme-1/_doc/1
{
  "@timestamp": "2021-05-28T15:33:39.333Z",
  "event": {
    "dataset": "nginx"
  }
}

# post example document to const keyword, I intentionally leave it _off_ the source document to maximize corner cases
# Otherwise you would add it like so:
# "data_stream": {
#   "dataset": "endpoint"
# }
DELETE const-keyword-frank-delme-1/_doc/1
POST const-keyword-frank-delme-1/_doc/1
{
  "@timestamp": "2021-05-28T15:33:39.333Z"
}

# Using fields works with an alias correctly with mixed const and non const keyword
GET const-keyword-frank-delme-1,keyword-frank-delme-1/_search
{
  "fields" : ["event.dataset"]
}

# Aggs returns both values regardless if you have added the const keyword to a _source or not
# This works with the alias above as well.
GET const-keyword-frank-delme-1,keyword-frank-delme-1/_search
{
  "size": 0,
  "aggs": {
    "event_dataset": {
      "terms": {
        "field": "event.dataset"
      }
    }
  }
}

# We can query with the alias and get both values back across alias
# and non-alias in combination with a const keyword and non const keyword
GET const-keyword-frank-delme-1,keyword-frank-delme-1/_search
{
  "fields" : ["event.dataset"],
  "query": {
    "terms": {
      "event.dataset": ["endpoint", "nginx"]
    }
  }
}

# Aggs returns both values regardless if you have added the const keyword to a _source or not
# This works with the alias above as well.
GET const-keyword-frank-delme-1,keyword-frank-delme-1/_search
{
  "size": 0,
  "aggs": {
    "event_dataset": {
      "terms": {
        "field": "event.dataset"
      }
    }
  }
}

Returns:

{
  "took" : 2,
  "timed_out" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 2,
      "relation" : "eq"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "event_dataset" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "endpoint",
          "doc_count" : 1
        },
        {
          "key" : "nginx",
          "doc_count" : 1
        }
      ]
    }
  }
}

# We can query with the alias and get both values back across alias
# and non-alias in combination with a const keyword and non const keyword
GET const-keyword-frank-delme-1,keyword-frank-delme-1/_search
{
  "fields" : ["event.dataset"],
  "query": {
    "terms": {
      "event.dataset": ["endpoint", "nginx"]
    }
  }
}

Returns:

{
  "took" : 4,
  "timed_out" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 2,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "const-keyword-frank-delme-1",
        "_id" : "1",
        "_score" : 1.0,
        "_source" : {
          "@timestamp" : "2021-05-28T15:33:39.333Z"
        },
        "fields" : {
          "event.dataset" : [
            "endpoint"
          ]
        }
      },
      {
        "_index" : "keyword-frank-delme-1",
        "_id" : "1",
        "_score" : 1.0,
        "_source" : {
          "@timestamp" : "2021-05-28T15:33:39.333Z",
          "event" : {
            "dataset" : "nginx"
          }
        },
        "fields" : {
          "event.dataset" : [
            "nginx"
          ]
        }
      }
    ]
  }
}

Note that you don't have the constant in the _source field and need to ensure you're using fields. You actually can add it to your _source document during ingest, that is allowed, but if you don't you have to use fields in order to get the value back if you want it back.