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btree/balance: add a diagram about divider cell assignment and some comments

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Jussi Saurio
2025-07-10 14:56:59 +03:00
parent 610b743f0d
commit 0d973d78a9
1 changed files with 57 additions and 4 deletions
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61
core/storage/btree.rs
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@@ -2776,9 +2776,60 @@ impl BTreeCursor {
let mut size_right_page = new_page_sizes[i];
let mut size_left_page = new_page_sizes[i - 1];
let mut cell_left = cell_array.cell_count_per_page_cumulative[i - 1] - 1;
// if leaf_data means we don't have divider, so the one we move from left is
// the same we add to right (we don't add divider to right).
let mut cell_right = cell_left + 1 - is_table_leaf as u16;
// When table leaves are being balanced, divider cells are not part of the balancing,
// because table dividers don't have payloads unlike index dividers.
// Hence:
// - For table leaves: the same cell that is removed from left is added to right.
// - For all other page types: the divider cell is added to right, and the last non-divider cell is removed from left;
// the cell removed from the left will later become a new divider cell in the parent page.
// TABLE LEAVES BALANCING:
// =======================
// Before balancing:
// LEFT RIGHT
// +-----+-----+-----+-----+ +-----+-----+
// | C1 | C2 | C3 | C4 | | C5 | C6 |
// +-----+-----+-----+-----+ +-----+-----+
// ^ ^
// (too full) (has space)
// After balancing:
// LEFT RIGHT
// +-----+-----+-----+ +-----+-----+-----+
// | C1 | C2 | C3 | | C4 | C5 | C6 |
// +-----+-----+-----+ +-----+-----+-----+
// ^
// (C4 moved directly)
//
// (C3's rowid also becomes the divider cell's rowid in the parent page
//
// OTHER PAGE TYPES BALANCING:
// ===========================
// Before balancing:
// PARENT: [...|D1|...]
// |
// LEFT RIGHT
// +-----+-----+-----+-----+ +-----+-----+
// | K1 | K2 | K3 | K4 | | K5 | K6 |
// +-----+-----+-----+-----+ +-----+-----+
// ^ ^
// (too full) (has space)
// After balancing:
// PARENT: [...|K4|...] <-- K4 becomes new divider
// |
// LEFT RIGHT
// +-----+-----+-----+ +-----+-----+-----+
// | K1 | K2 | K3 | | D1 | K5 | K6 |
// +-----+-----+-----+ +-----+-----+-----+
// ^
// (old divider D1 added to right)
// Legend:
// - C# = Cell (table leaf)
// - K# = Key cell (index/internal node)
// - D# = Divider cell
let mut cell_right = if is_table_leaf {
cell_left
} else {
cell_left + 1
};
loop {
let cell_left_size = cell_array.cell_size_bytes(cell_left as usize) as i64;
let cell_right_size =
@@ -2946,7 +2997,9 @@ impl BTreeCursor {
// * first overflow page (u32 optional)
new_divider_cell.extend_from_slice(&divider_cell[4..]);
} else if is_table_leaf {
// Leaf table
// For table leaves, divider_cell_idx effectively points to the last cell of the old left page.
// The new divider cell's rowid becomes the second-to-last cell's rowid.
// i.e. in the diagram above, the new divider cell's rowid becomes the rowid of C3.
// FIXME: not needed conversion
// FIXME: need to update cell size in order to free correctly?
// insert into cell with correct range should be enough