presto-elasticsearch/src/main/java/io/prestosql/elasticsearch/ElasticsearchPageSource.java/ElasticsearchPageSource/flattenFields
class ElasticsearchPageSource:    private Map<String, Type> flattenFields(List<ElasticsearchColumnHandle> columns)
    {
        Map<String, Type> result = new HashMap<>();

        for (ElasticsearchColumnHandle column : columns) {
            flattenFields(result, column.getName(), column.getType());
        }

        return result;
    }