@Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        if (columnName == null || columnName.isEmpty()) {
            builder.append("column").append(columnIndex);
        } else {
            builder.append(columnName);
        }
        return builder.append(' ').append(originalTypeName).toString();
    }
Gets aggregate function when column type is
{@link ClickHouseDataType#AggregateFunction}. So it will return
{@link ClickHouseAggregateFunction#quantile} when the column type is
{@code AggregateFunction(quantiles(0.5, 0.9), UInt64)}.

@return function name, null when column type is not AggregateFunction
clickhouse-data/src/main/java/com/clickhouse/data/ClickHouseColumn.java