code: public function queryMap($key = 0, $value = 1)
  	{
  		$fetchMode = $this->resolveFetchMode(is_string($key) || is_string($value));
  		$result = $this->execute();
  		$map = [];
  		
  		try
  		{
  			while ($row = $result->fetch($fetchMode))
  			{
  				if (!isset($row[$key]) || !key_exists($value, $row)) 
  					throw new MySqlException(
  						"Key '$key' or Value '$value' columns not found in the query result: " . 
  						implode(array_keys($row)));
  				
  				$map[$row[$key]] = $row[$value];
  			}
  		}
  		// Free resources when generator released before reaching the end of the iteration.
  		finally
  		{
  			$result->closeCursor();
  		}
  		
  		return $map;
  	}
docs: Return an array where the result of one column is the index and the second is value.
  @param int|string $key Name of the key column.
  @param int|string $value Name of the value column
  @return array|false