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Fix existing table btree backwards iteration logic

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This commit is contained in:
Jussi Saurio
2025-04-08 17:05:52 +03:00
parent d9bae633c0
commit 0bb87b060a
1 changed files with 49 additions and 18 deletions
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67
core/storage/btree.rs
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@@ -431,8 +431,7 @@ impl BTreeCursor {
// todo: find a better way to flag moved to end or begin of page
if self.stack.current_cell_index_less_than_min() {
loop {
if self.stack.current_cell_index() > 0 {
self.stack.retreat();
if self.stack.current_cell_index() >= 0 {
break;
}
if self.stack.has_parent() {
@@ -448,11 +447,6 @@ impl BTreeCursor {
}
let cell_idx = cell_idx as usize;
tracing::trace!(
"get_prev_record current id={} cell={}",
page.get().id,
cell_idx
);
return_if_locked!(page);
if !page.is_loaded() {
self.pager.load_page(page.clone())?;
@@ -468,8 +462,7 @@ impl BTreeCursor {
let rightmost_pointer = contents.rightmost_pointer();
if let Some(rightmost_pointer) = rightmost_pointer {
self.stack
.push(self.pager.read_page(rightmost_pointer as usize)?);
self.stack.set_cell_index(i32::MAX);
.push_backwards(self.pager.read_page(rightmost_pointer as usize)?);
continue;
}
}
@@ -480,7 +473,6 @@ impl BTreeCursor {
} else {
cell_idx
};
let cell = contents.cell_get(
cell_idx,
payload_overflow_threshold_max(contents.page_type(), self.usable_space() as u16),
@@ -494,9 +486,7 @@ impl BTreeCursor {
_rowid,
}) => {
let mem_page = self.pager.read_page(_left_child_page as usize)?;
self.stack.push(mem_page);
// use cell_index = i32::MAX to tell next loop to go to the end of the current page
self.stack.set_cell_index(i32::MAX);
self.stack.push_backwards(mem_page);
continue;
}
BTreeCell::TableLeafCell(TableLeafCell {
@@ -523,6 +513,14 @@ impl BTreeCursor {
payload_size,
}) => {
if !self.going_upwards {
// In backwards iteration, if we haven't just moved to this interior node from the
// right child, but instead are about to move to the left child, we need to retreat
// so that we don't come back to this node again.
// For example:
// this parent: key 666
// left child has: key 663, key 664, key 665
// we need to move to the previous parent (with e.g. key 662) when iterating backwards.
self.stack.retreat();
let mem_page = self.pager.read_page(left_child_page as usize)?;
self.stack.push(mem_page);
// use cell_index = i32::MAX to tell next loop to go to the end of the current page
@@ -538,7 +536,7 @@ impl BTreeCursor {
)?
};
// Going upwards = we just moved to an interior cell from a leaf.
// Going upwards = we just moved to an interior cell from the right child.
// On the first pass we must take the record from the interior cell (since unlike table btrees, index interior cells have payloads)
// We then mark going_upwards=false so that we go back down the tree on the next invocation.
self.going_upwards = false;
@@ -1206,6 +1204,13 @@ impl BTreeCursor {
// 6. If we find the cell, we return the record. Otherwise, we return an empty result.
self.move_to_root();
let iter_dir = match self.iteration_state {
IterationState::Iterating(IterationDirection::Backwards) => {
IterationDirection::Backwards
}
_ => IterationDirection::Forwards,
};
loop {
let page = self.stack.top();
return_if_locked!(page);
@@ -1284,12 +1289,22 @@ impl BTreeCursor {
cmp, self.iteration_state
),
};
self.stack.advance();
if target_leaf_page_is_in_left_subtree {
// If we found our target rowid in the left subtree,
// we need to move the parent cell pointer forwards or backwards depending on the iteration direction.
// For example: since the internal node contains the max rowid of the left subtree, we need to move the
// parent pointer backwards in backwards iteration so that we don't come back to the parent again.
// E.g.
// this parent: rowid 666
// left child has: 664,665,666
// we need to move to the previous parent (with e.g. rowid 663) when iterating backwards.
self.stack.next(iter_dir);
let mem_page = self.pager.read_page(*_left_child_page as usize)?;
self.stack.push(mem_page);
found_cell = true;
break;
} else {
self.stack.advance();
}
}
BTreeCell::TableLeafCell(TableLeafCell {
@@ -1392,7 +1407,15 @@ impl BTreeCursor {
),
};
if target_leaf_page_is_in_left_subtree {
// we don't advance in case of index tree internal nodes because we will visit this node going up
// we don't advance in case of forward iteration and index tree internal nodes because we will visit this node going up.
// in backwards iteration, we must retreat because otherwise we would unnecessarily visit this node again.
// Example:
// this parent: key 666, and we found the target key in the left child.
// left child has: key 663, key 664, key 665
// we need to move to the previous parent (with e.g. key 662) when iterating backwards so that we don't end up back here again.
if iter_dir == IterationDirection::Backwards {
self.stack.retreat();
}
let mem_page = self.pager.read_page(*left_child_page as usize)?;
self.stack.push(mem_page);
found_cell = true;
@@ -3816,7 +3839,7 @@ impl PageStack {
}
/// Push a new page onto the stack.
/// This effectively means traversing to a child page.
fn push(&self, page: PageRef) {
fn _push(&self, page: PageRef, starting_cell_idx: i32) {
tracing::trace!(
"pagestack::push(current={}, new_page_id={})",
self.current_page.get(),
@@ -3829,7 +3852,15 @@ impl PageStack {
"corrupted database, stack is bigger than expected"
);
self.stack.borrow_mut()[current as usize] = Some(page);
self.cell_indices.borrow_mut()[current as usize] = 0;
self.cell_indices.borrow_mut()[current as usize] = starting_cell_idx;
}
fn push(&self, page: PageRef) {
self._push(page, 0);
}
fn push_backwards(&self, page: PageRef) {
self._push(page, i32::MAX);
}
/// Pop a page off the stack.