public function getEnumValues($tableName, $columnName) {

		// check whether column exists

		if(!$this->columnExists($tableName, $columnName)) {
			throw new \PDOException(sprintf("Unknown column '%s' in table '%s'.", $columnName, $tableName));
		}
		
		// wrong data type
		
		$dataType = $this->tableStructureCache[$tableName][$columnName]['dataType']; 
		
		if(!($dataType === 'enum' || $dataType === 'set')) {
			throw new \PDOException(sprintf("Column '%s' in table '%s' is not of type ENUM or SET.", $columnName, $tableName));
		}
		
		// extract enum values the first time

		if(!isset($this->tableStructureCache[$tableName][$columnName]['enumValues'])) {
			preg_match_all(
				"~'(.*?)'~i",
				$this->tableStructureCache[$tableName][$columnName]['columnType'],
				$matches
			);
			
			$this->tableStructureCache[$tableName][$columnName]['enumValues'] = $matches[1];
		}
		
		return $this->tableStructureCache[$tableName][$columnName]['enumValues'];
		
	}