chore: add comments

src/mito2/src/read/series_scan.rs

@@ -464,6 +464,7 @@ impl SenderList {
 
     /// Finds a partition and sends the batch to the partition.
     async fn send_batch(&mut self, mut batch: SeriesBatch) -> Result<()> {
+        // Sends the batch without blocking first.
         match self.try_send_batch(batch)? {
             Some(b) => {
                 // Unable to send batch to partition.

src/query/src/optimizer/parallelize_scan.rs

@@ -67,12 +67,13 @@ impl ParallelizeScan {
                         return Ok(Transformed::no(plan));
                     }
 
-                    // don't parallelize if we want per series distribution
                     if matches!(
                         region_scan_exec.distribution(),
                         Some(TimeSeriesDistribution::PerSeries)
                     ) {
                         let partition_range = region_scan_exec.get_partition_ranges();
+                        // HACK: Allocate expected_partition_num empty partitions to indicate
+                        // the expected partition number.
                         let mut new_partitions = vec![vec![]; expected_partition_num];
                         new_partitions[0] = partition_range;
                         let new_plan = region_scan_exec