Page Layout
Each data page is divided into the following sections:
- Page Header
- Item ID Array (Slot Directory)
- Free Space
- Tuple Data
The page uses a slotted-page layout inspired by PostgreSQL, where:
- the Item ID array grows forward from the beginning of the page,
- tuple data grows backward from the end of the page,
- and free space exists between them.
| Component | Description |
|---|---|
| Page Header | Stores the lower and upper offsets used for page space management. |
| lower | Points to the end of the Item ID array (slot directory). |
| upper | Points to the beginning of tuple data. |
| Item ID Array (Slot Directory) | Grows forward from the page header and stores metadata about tuples. |
| Slot Entry Size | Each slot entry is 8 bytes. |
| Slot Structure | offset → 4 bytes, length → 4 bytes |
| Slot Storage | Slots are stored sequentially immediately after the page header. |
| Tuple Count Calculation | (lower - PAGE_HEADER_SIZE) / ITEM_ID_SIZE |
| Tuple Data | Actual tuple/record bytes stored from the end of the page backward. |
| Free Space | Space available between the Item ID array and tuple data. |
| Free Space Calculation | upper - lower |
| Implementation Location | src/backend/page/mod.rs |
Note: The current page layout is inspired by PostgreSQL. For reference, the PostgreSQL page layout is described at:
https://www.postgresql.org/docs/current/storage-page-layout.html#STORAGE-PAGE-LAYOUT-FIGURE