Adding suppport for reverse iteration

[ethe]

In current, Transaction::scan and DB::scan only supports ascending order, it should be better to support reverse order iteration. This includes two steps tasks:

• API design
• the reverse iteration implentation

[vaibhawvipul]

Taking this up

[vaibhawvipul]

This is what I am thinkging, please let me know if this is okay or feel free to mention changes.

Api design -

enum ScanOrder {
    Ascending,
    Descending,

This can be passed to DB::Scan

pub async fn scan<'scan, T: 'scan>(
        &'scan self,
        range: (Bound<&'scan R::Key>, Bound<&'scan R::Key>),
        mut f: impl FnMut(TransactionEntry<'_, R>) -> T + 'scan,
        order: ScanOrder
    ) -> impl Stream<Item = Result<T, CommitError<R>>> + 'scan

and

Also to Transaction::Scan

pub async fn scan<'scan>(
       &'scan self,
       range: (Bound<&'scan R::Key>, Bound<&'scan R::Key>),
       order: ScanOrder
   ) -> Scan<'scan, R, FP> {
       let streams = vec![TransactionScan {
           inner: self.local.range(range), // match here to wrt to scanorder to do ascending(default) or descending
           ts: self.ts,
       }
       .into()];
       Scan::new(&self.share, range, self.ts, &self.version, streams)
   }

[ethe]

Is it better to have a builder pattern here? Because I think iteration is much more common than reverse iteration, we could keep:

pub async fn scan<'scan>(
       &'scan self,
       range: (Bound<&'scan R::Key>, Bound<&'scan R::Key>),
   ) -> Scan<'scan, R, FP>

then we add reverse method on Scan, users are able to use it like

            let mut scan = txn
                .scan((Bound::Included(&name), Bound::Excluded(&upper)))
                .await
                .reverse()
                .take()
                .await
                .unwrap();

In this way, if users do not explicitly declare reverse iterating, Tonbo could use iterating as default.

[vaibhawvipul]

Aligned.

[KKould]

MergeScan consists of multiple iterators

• MutableScan
• ImmutableScan
• LevelStream
• SsTableScan
  • ParquetRecordBatchStream

These should all implement reverse