---

Identifying deleted records on disks

---

• A caveat when using physical addresses to reference a block/record
  • Consider 2 block with physical database records referencing a record Y:

    ---

    ---

  • Consider the situation when the record Y is deleted :

    Observation:

    The physical addresses will reference an incorrect (bogus) record !!!

  ---

  ---

  ---

  ---

• Techniques to handle record deletion
  • Handling record deletion using logical addresses (it is pretty easy):

    Before deleting record Y: After deleting record Y: Very important: The logical address used by record Y must remain in the map table !!! I.e.: logical address used by record Y cannot be re-used !!!

    ---

    ---

    ---

    ---

  • Handling record deletion using physical addresses:

    Use a tomb stone record.....

    ---

    ---

  • Tomb stone record

    Tomb stone record: a (very small) special purpose record used to indicate a deleted record When a record is delete, it is replaced by the tomb stone record

    Example:

    Before deleting record Y: After deleting record Y:

  ---

  ---

  ---

  ---

• Caveat using a tomb stone record
  • Insertion:

    When you insert a new record: You cannot use the space of a tomb stone record !!! Because otherwise there will be an error: Existing record references to the deleted record will now reference to the newly inserted trecord: