storage: support range-filter scan by sort key

[skyzh]

Our storage engine already supports sorting by sort key. To leverage the property in storage, we can provide a range_scan interface on storage, so as to support range filter scan for executors.

To support range_scan, we should be able to start scanning from a sort key in storage engine. This requires two steps:

• record sort key in block index
• support create iterator by sort key
• add interface for executors' use

Currently, the first_key field is empty in block builder. We should record first_key in order to support seek. Note that #588 will add composite sort key support. For this issue, we may only support range filter scan by the first element in composite sort key.

risinglight/src/storage/secondary/block/block_index_builder.rs

Lines 39 to 54 in 7c5e0d5

	pub fn finish_block(
	&mut self,
	block_type: BlockType,
	column_data: &mut Vec<u8>,
	block_data: &mut Vec<u8>,
	stats: Vec<BlockStatistics>,
	) {
	self.indexes.push(BlockIndex {
	offset: column_data.len() as u64,
	length: block_data.len() as u64 + BLOCK_HEADER_SIZE as u64,
	first_rowid: self.last_row_count as u32,
	row_count: (self.row_count - self.last_row_count) as u32,
	/// TODO(chi): support sort key
	first_key: "".into(),
	stats,
	});

After that, we should be able to support create RowSetIterator by sort key.

risinglight/src/storage/secondary/rowset/rowset_iterator.rs

Lines 27 to 33 in 7c5e0d5

	pub async fn new(
	rowset: Arc<DiskRowset>,
	column_refs: Arc<[StorageColumnRef]>,
	dvs: Vec<Arc<DeleteVector>>,
	seek_pos: ColumnSeekPosition,
	expr: Option<BoundExpr>,
	) -> StorageResult<Self> {

The interface should have two new fields start_key: Option<Vec<u8>> and end_key: Option<Vec<u8>>. RowSetIterator should return rows only in this range. At the same time, the RowSetIterator should directly seek to the block of start_key instead of scanning from the beginning.

When everything's ready, we can finally expose range-filter scan interface to the compute layer. This requires us to modify:

risinglight/src/storage/secondary/transaction.rs

Lines 356 to 364 in 7c5e0d5

	fn scan<'a>(
	&'a self,
	begin_sort_key: Option<&'a [u8]>,
	end_sort_key: Option<&'a [u8]>,
	col_idx: &'a [StorageColumnRef],
	is_sorted: bool,
	reversed: bool,
	expr: Option<BoundExpr>,
	) -> Self::ScanResultFuture<'a> {