Provides metadata about columns for data visualization.

        :return: dict, with the fields name, type, is_date, is_dim and agg.

def columns(self):
        """Provides metadata about columns for data visualization.

        :return: dict, with the fields name, type, is_date, is_dim and agg.
        """
        if self.df.empty:
            return None

        columns = []
        sample_size = min(INFER_COL_TYPES_SAMPLE_SIZE, len(self.df.index))
        sample = self.df
        if sample_size:
            sample = self.df.sample(sample_size)
        for col in self.df.dtypes.keys():
            db_type_str = (
                self._type_dict.get(col) or
                self.db_type(self.df.dtypes[col])
            )
            column = {
                'name': col,
                'agg': self.agg_func(self.df.dtypes[col], col),
                'type': db_type_str,
                'is_date': self.is_date(self.df.dtypes[col], db_type_str),
                'is_dim': self.is_dimension(self.df.dtypes[col], col),
            }

            if not db_type_str or db_type_str.upper() == 'OBJECT':
                v = sample[col].iloc[0] if not sample[col].empty else None
                if isinstance(v, str):
                    column['type'] = 'STRING'
                elif isinstance(v, int):
                    column['type'] = 'INT'
                elif isinstance(v, float):
                    column['type'] = 'FLOAT'
                elif isinstance(v, (datetime, date)):
                    column['type'] = 'DATETIME'
                    column['is_date'] = True
                    column['is_dim'] = False
                # check if encoded datetime
                if (
                        column['type'] == 'STRING' and
                        self.datetime_conversion_rate(sample[col]) >
                        INFER_COL_TYPES_THRESHOLD):
                    column.update({
                        'is_date': True,
                        'is_dim': False,
                        'agg': None,
                    })
            # 'agg' is optional attribute
            if not column['agg']:
                column.pop('agg', None)
            columns.append(column)
        return columns