docs: private Optional<String> validateProctimeAttribute(Optional<String> proctimeAttribute) {
  		return proctimeAttribute.map((attribute) -> {
  			// validate that field exists and is of correct type
  			Optional<TypeInformation<?>> tpe = schema.getFieldType(attribute);
  			if (!tpe.isPresent()) {
  				throw new ValidationException("Processing time attribute '" + attribute + "' is not present in TableSchema.");
  			} else if (tpe.get() != Types.SQL_TIMESTAMP()) {
  				throw new ValidationException("Processing time attribute '" + attribute + "' is not of type SQL_TIMESTAMP.");
  			}
  			return attribute;
  		}