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Delete Tuple

Delete Tuple​

Description:
Deletes a tuple from a heap file by marking its slot entry as deleted. The tuple is not physically removed from the page; instead, its slot directory entry is invalidated, allowing future maintenance operations (e.g., VACUUM) to reclaim space.

Function:

pub fn delete_tuple(
    &mut self,
    page_id: u32,
    slot_id: u32,
) -> io::Result<u32>

Input:

  • page_id — Identifier of the page containing the tuple.
  • slot_id — Slot identifier of the tuple within the page.

Output:

  • Returns the number of bytes freed (u32) on success.
  • Returns an io::Error if the page or slot is invalid, or if a page read/write operation fails.

Implementation:

  1. Validate that page_id is within the heap file's page range.
  2. Read the target page from disk.
  3. Retrieve the tuple's slot directory entry.
  4. Compute the number of bytes freed: 
    tuple_length + ITEM_ID_SIZE
  5. Mark the slot as deleted by setting: 
    offset = 0
    length = 0
  6. Read the page's lower and upper pointers.
  7. If the deleted tuple occupies the current upper boundary, reclaim contiguous data space by adjusting upper.
  8. If the deleted slot is the last slot entry in the slot directory, reclaim slot array space by adjusting lower.
  9. Write the modified page back to disk.
  10. Decrement the heap file's tuple count.
  11. Persist the updated heap header.
  12. Return the number of freed bytes.

Internal API Calls:

  • read_page(&mut self.file_handle, &mut page, page_id)

    • Loads the target page from disk into memory.

  • get_slot_entry(&page, slot_id)

    • Retrieves the tuple offset and length from the slot directory.

  • get_tuple_count(&page)

    • Returns the number of slot entries in the page.

  • write_page(&mut self.file_handle, &mut page, page_id)

    • Persists the modified page back to disk.

  • update_header_page(&mut self.file_handle, &self.header)

    • Writes the updated heap header metadata to disk.

Files Created/Modified:

  • Heap table file containing the target page.
  • Heap header page.

Storage Updates:

  • Marks the tuple's slot directory entry as deleted: 
    offset = 0
    length = 0
  • Updates page metadata (lower and/or upper) when contiguous space can be reclaimed.
  • Decrements: 
    self.header.total_tuples
  • Updates the heap header on disk.

Notes:

  • The tuple is logically deleted but not physically removed from the page.
  • Free Space Map (FSM) entries are not updated during deletion because contiguous free space does not necessarily increase.
  • Space is fully reclaimed later through page compaction or VACUUM operations.