- A record Y can be
referenced by
other records
(here: records X1 and X2):
- Consider
the situation
when the record Y is
deleted
:
Observation:
- The references will refer an incorrect (bogus) record !!!
- Fact:
- The delete precedure is
the
same for
a sorted or
unsorted file
- The procedure
will differ:
- When records are
referenced using:
- a physical database (record) address or
- a logical database (record) address
- When using
a physical database (record)
address, we can use
different
referencing methods:
- Record address = block address + offset in block
- Record address = block address + index in offset table
- When records are
referenced using:
Recall: you can reference a record (inside a block) using:
- block database address +
offset in
the block
- block database address +
index in
the (block)
offset table
- The delete precedure is
the
same for
a sorted or
unsorted file
- Deleting a
record when
record references
uses logical addresses:
- Before deleting record Y
(that is referenced by
records X1 and X2):
After deleting record Y:
Deleted record is identified by a NULL physical address in the Map table
- Before deleting record Y
(that is referenced by
records X1 and X2):
- Very important:
- The logical address used by
record Y
must
remain in the
map table !!!
Furthermore:
- The logical address used by record Y cannot be re-used !!!
- The logical address used by
record Y
must
remain in the
map table !!!
- Delete procedure
when records are
referenced by
(block database address +
index in
offset table):
- Before the deletion:
- After the deletion:
set the
offset in
offset table to
NULL
An NULL offset is illegal
This value represents a tombstone for a deleted record
- Before the deletion:
- Delete procedure
when records are
referenced by
(block database address +
(absolute) offset):
- Before the deletion:
- After the deletion:
replace the
deleted record with a
special tombstone record
Note:
- The other records
are usually not
moved
(to "save space")
because:
- We will then need to update other database addresses that reference to these (moved) records !!!
- The other records
are usually not
moved
(to "save space")
because:
- Before the deletion:
- 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:
- Tomb stone record:
a (very small)
special purpose record
used to indicate a deleted record
- Insertion:
-
When you insert a
new record:
- You
cannot
re-use the
disk space
of a
tomb stone record !!!
- The tomb stone record must be preserved !!!
Because otherwise there will be an error:
- There are still
references to
the deleted record
Y:
- If you
re-use
the disk space for
the record
Y for
some record
Z, then:
- The existing
record references
to the deleted record Y will
now
reference to the
newly inserted record Z
(And will access the wrong record data !!!)
Graphically:
- The existing
record references
to the deleted record Y will
now
reference to the
newly inserted record Z
- You
cannot
re-use the
disk space
of a
tomb stone record !!!
-
When you insert a
new record: