Function downsample 

pub fn downsample(
    batch: &RecordBatch,
    value_column: &str,
    interval: i64,
    function: &AggFn,
    memory_tracker: Option<&MemoryTracker>,
) -> Result<RecordBatch>

Downsample a RecordBatch into fixed-width time buckets.

For each bucket [bucket_start, bucket_start + interval), the aggregation function is applied to the values in the value_column.

Returns a new RecordBatch with two columns: _time (bucket start) and the aggregated value column.

Arguments

• batch – input data (must have a _time Int64 column)
• value_column – name of the column to aggregate
• interval – bucket width in the same unit as timestamps (e.g. nanos)
• function – aggregation function to apply per bucket

Errors

Returns an error if columns are missing or have unexpected types.