Fielddata error indexing string fields

[iamtheschmitzer (Jeff Schmitz)]

In notes of Part 5 of crash course I see

By default, every string gets mapped twice as a text field and as a keyword multi-field. Each field type is primed for different types of requests.

However, when I try to aggregate by a string field (with a limited set of single-word values) I get an error:

      {
        "type" : "illegal_argument_exception",
        "reason" : "Text fields are not optimised for operations that require per-document field data like aggregations and sorting, so these operations are disabled by default. Please use a keyword field instead. Alternatively, set fielddata=true on [measure] in order to load field data by uninverting the inverted index. Note that this can use significant memory."
      }

What does this error mean and what is the resolution? For reference my query looks like this:

GET index_name/_search
{
  "aggs": {
    "maxtime by measure": {
      "terms": {
        "field": "measure"
        }
      },
      "aggs": {
        "max": {
          "field": "time"
        }
      }
  },
  "size": 0
}

Thanks in advance

[mayya (Mayya Sharipova)]

When you index a string and don't provide a mapping, measure will be indexed as a text field, and measure.keyword as a keyword field. So you need to run your terms aggregation on measure.keyword field.

[iamtheschmitzer (Jeff Schmitz)]

Thanks, very helpful to know