- Inserting into a file
of unordered records is
trivial:
Find a used block that has sufficient space to hold the insert record if ( block NOT found ) { get a new block; } if ( no block available ) { return error } insert record;
- Inserting into a file
of
ordered records
may need to:
- Move some records !!!
Graphically:
- Before the insert:
- After the insert:
- Recall:
referencing to
other records
- A record can be
referenced by
another record
Example: record Y is referenced by records X1 and X2:
- A record can be
referenced by
another record
- Result of
moving records
when using
physical database addresses:
- Records X1 and X2 references
a record Y using
physical addresses:
- When
record Y is
moved:
- We must update all physical addresses referencing to record Y
Graphically:
(We will need to maintain a data structure that store the location of all the referencing database addresses !!!)
- Records X1 and X2 references
a record Y using
physical addresses:
- Easier solution:
use logical database addresses !!!
- We can easily
solve the
record movement problem
using
logical database address:
See: click here
- We can easily
solve the
record movement problem
using
logical database address:
- More problem: overflow
- There is no more space
in the block
where the record
must be
inserted:
- There is no more space
in the block
where the record
must be
inserted:
- Commonly used solution:
overflow blocks
- The record is
inserted and
pushes an
overflow record out:
- An overflow block
is allocated and
linked by
an overflow block pointer
field in the
block header:
- The overflow record is stored
in the overflow block:
-
The offset
must be changed to
a forwording index:
- The record is
inserted and
pushes an
overflow record out:
- Note:
- The forwarding index can consist
of ((one-bit) flag + index in offset table)
- Flag is used as follows:
- If flag = 0,
then the index is used with the
offset table in the
current block
- If flag = 1, then the index is used with the offset table in the next overflow block
- If flag = 0,
then the index is used with the
offset table in the
current block
- The forwarding index can consist
of ((one-bit) flag + index in offset table)