public String getFunction() {
        return dataType == ClickHouseDataType.AggregateFunction ? parameters.get(0) : null;
    }
Gets function when column type is
{@link ClickHouseDataType#AggregateFunction}. So it will return
{@code quantiles(0.5, 0.9)} when the column type is
{@code AggregateFunction(quantiles(0.5, 0.9), UInt64)}.

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