presto-cassandra/src/main/java/io/prestosql/plugin/cassandra/CassandraType.java/CassandraType/buildMapValue
class CassandraType:    private static String buildMapValue(Map<?, ?> map, DataType keyType, DataType valueType)
    {
        StringBuilder sb = new StringBuilder();
        sb.append("{");
        for (Map.Entry<?, ?> entry : map.entrySet()) {
            if (sb.length() > 1) {
                sb.append(",");
            }
            sb.append(objectToString(entry.getKey(), keyType));
            sb.append(":");
            sb.append(objectToString(entry.getValue(), valueType));
        }
        sb.append("}");
        return sb.toString();
    }