storage: support composite sort key in MergeIterator

[skyzh]

Currently, MergeIterator only supports taking a single sort_idx as parameter:

risinglight/src/storage/secondary/merge_iterator.rs

Lines 34 to 36 in 7c5e0d5

	impl MergeIterator {
	pub fn new(iters: Vec<SecondaryIterator>, sort_key_idx: usize) -> Self {
	Self {

We should make MergeIterator be able to accept a vector of sort_idx. For example, sort_key_idx = [2, 1, 0]:

• Implies that the child iterators are already being sorted using 3 columns: firstly be column #2, then by column #1, at last by #0.
  *MergeIterator should also sort by this rule.

pub fn new(iters: Vec<SecondaryIterator>, sort_key_idx: Vec<usize>) -> Self {

After we finish the refactor in the storage layer, we can firstly make all places to pass vec![sort_key] instead of sort_key. Then, we can refactor little by little to support full composite sort key support across our system.

[BaymaxHWY]

Hi, can i try this? This requirement just changes the code about sort_key_idx and the function compare_data()?

[skyzh]

Hi, can i try this? This requirement just changes the code about sort_key_idx and the function compare_data()?

Yes, I think so.