| (a) |
In deciding which type of file
organization should be employed in a system, a designer has to take into account a number
of factors. Explain each of the following factors which may have to be considered : |
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(i) Volatility (ii) Run integration
(iii) Access type
(iv) Hit rate
(v) Life expectancy |
[2] [2]
[2]
[2]
[2] |
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(i)
- Volatility is concerned with changes
made to a file over a period of time.
- All files held on direct access
devices are adversely affected by high volatility because of synonyms, which degrade
performance.
(ii)
- Direct processing allows a number of
runs to be consolidated into a single run, this is termed run integration.
- Run integration depends upon the
ability of direct or indexed sequential files to handle transactions in any order.
(iii)
- For sequential access, a sequential
organized file is appropriate.
- For a reduced hit rate, an indexed
sequential file is appropriate.
- If the hit rate is reduced
drastically, a direct access file is appropriate.
(iv)
- The hit rate refers to the number of
records that are retrieved for reference or update purposes.
- Sequential files are suitable for
high hit rates, while direct access files are suitable for low hit rates.
(v)
- A temporary file may be designed for
convenience rather than optimal performance.
- By the time the feasibility study
has been concluded, the need for the file may no loner exist.
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| (b) |
Describe the use of normal
and overflow track entries in relation to Indexed Sequential Files. |
[6] |
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- Normal entry keeps track of the
highest key in the track.
- In normal entry, the highest key on
the prime data track changes as records are inserted.
- In normal entry, the home track
address does not change.
- Overflow entry links the highest key
with the first record in the chain of overflow records for that track.
- In overflow entry, the highest key
on the logical track does not change.
- In overflow entry, the address of
the first overflow in chain changes as records overflow.
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| (c) |
Name two structures that can
adopted by Indexed Sequential Files. |
[2] |
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- Index area.
- Home data area.
- Overflow area.
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| (d) |
Give two advantages of Indexed
Sequential Files. |
[2] |
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- Transactions may be stored or
unsorted.
- Only the affected master records are
processed during updating.
- The response time is reasonably
fast.
- They facilitate file enquiry.
- They can be processed sequentially
or randomly.
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