1. File and Database Concepts

2. Records Fixed or variable length records: pros and cons?
“A record is an organised collection of information about an object or item, with a unique identifier or key.”
Ted Postlethwaite 10 West Hill Highgate London N6 3PY
Fixed or variable length records: pros and cons?
Ted
Postlethwaite
10 West Hill
Highgate
London
N6 3PY
Ted
Postlethwaite
10 West Hill
Highgate
London
N6 3PY

3. Pros and Cons: Fixed and Variable Length Records
Fixed-Length Records
Easy to implement
Can predict where records will start and end so support for direct access
Wastes space
Can be inflexible as space allocated may become too small
Variable-Length Records
Uses disk space economically
Flexible
Difficult to implement
Sequential access only

4. Key fields
A key field is a piece of data that uniquely identifies a record
Fields like surname or date of birth are not sufficient because they are not necessarily unique
Most systems create a random number to serve as a key
Advanced database systems will detect immediately if a key is not unique

5. Unordered Sequential Access
Records not in order.
Search starts at the beginning of the file.
Records read in the order that they are stored. ►
Harris
Shah
Oworu
Tooey
Wickham
Hussein
Heim
Hansen
Schmidt
Arnold
O’Hanlon
Zachary
Christopher

6. Ordered Sequential Access
Records are in some order
Search starts at the beginning of the file ►
Arnold
Christopher
Hansen
Harris
Heim
Hussein
O’Hanlon
Oworu
Schmidt
Shah
Tooey
Wickham
Zachary

7. Ordered vs Unordered?
Imagine you have a two files, each of 100,000 surnames, many of which are repeated.
One file is unordered, the other is alphabetical.
You want to find all occurrences of the name “Smith”.

8. Five-Minute Task How much of the unordered file do you have to search?
How much of the ordered file do you have to search?
Geek question: In terms of search time, how much more efficient are ordered sequential files than unordered sequential files on average in this type of search?

9. Five-Minute Task Feedback: Unordered File
With an unordered file you would always have to search all of the file.
Why? Because you have no way of knowing that the last record of the file isn’t a “Smith”!

10. Five-Minute Task Feedback: Ordered File
You would only have to search until the end of the “Smiths”
Why? Because the file is in order, you know there are no more “Smiths” to look for.

11. Five-Minute Task Feedback: Geek Question
Ordered files would be approximately twice as efficient on average.
Why? Of 100,000 sequenced records, you would sometimes need to search 1 records, sometimes 2, sometimes 3… sometimes 99,999, sometimes 100,000. The average of 1 … 100,000 is (100,001 / 2) ≈ 50,000. So on average you would have to search 50,000 records to find the block you want. With an unordered file we know we always have to search all 100,000 records. That’s twice as many as the ordered file, so an ordered file is twice as efficient for this type of search!

12. Direct Access Also called random access
Direct access to a single record with no need to search.
Hashing algorithm creates disk address from record key.
Harris
Shah
Oworu
Tooey
Wickham
Hussein
Heim
Hansen
Schmidt
Arnold
O’Hanlon
Zachary
Christopher
Hashing Algorithm
Hansen?
“Collision” occurs when hashing algorithm tries to put two things in the same place!

13. Dealing with Collisions
Hashing algorithm gives the address at which to store the record.
Overflow to next address if target address is full.
Harris
Shah
Oworu
Tooey
Wickham Hussein
Heim
Schmidt
Arnold
O’Hanlon
Zachary
Christopher
Hansen
‘Bucket’
Hashing Algorithm
Hansen
FULL
Performance deteriorates over time as more overflows happen.
Overflow

14. How Hashing Works There are lots of ways, but we’ll look at N Mod M.
Modulo (Mod) is a mathematical operation like , , , , which gives the remainder when one number is divided by another.
Question: What is the highest possible remainder when a number is divided by M?

15. N Mod M Mod 11 We have 11 buckets, so M = 11
1243 1
1244 2
1234 3
1235 4
1236 5
1237 6
1238 7
1239 8
1240 9
1241 10
1242
We have 11 buckets, so M = 11
1234
1235
1236
1237
Mod 11
1238
Record Keys
1239
Buckets
1240
1241
1242
1243
1234 Mod 11 = 2, so the record with the key 1234 goes into bucket number 2.
1244

16. Indexed Sequential Wickham? Harris Shah Oworu Tooey Wickham Hussein
Heim
Hansen
Schmidt
Arnold
O’Hanlon
Zachary
Christopher
Hashing Algorithm
Supports direct access and sequential access.
A table with more than one column can have more than one index.
A very large index may have its own index (multi-level index).
INDEX

17. Another method of direct access
Let's say we use fixed-length records
Each record is 128 bytes long
I know the memory address of the first record
How do I get to the nth record?
Harris
Shah
Oworu
Tooey
Wickham
Hussein
Heim
Hansen
Schmidt
Arnold
O’Hanlon
Zachary
Christopher
I access the first record with an offset of 128n

18. The Power of Indexes From the phone book Contestant number one find…
Ibay, Aileen M
Contestant number two find…

19. Partially indexed files
The file is ordered
The index contains an entry of every nth record
To access a record you sequentially search the index for the key just before the key you are seeking
Then follow the index pointer to the memory address in the middle of the file
Then search the file sequentially for the correct record
Arnold
Heim
Schmidt
Arnold
Christopher
Hansen
Harris
Heim
Hussein
O’Hanlon
Oworu
Schmidt
Shah
Tooey
Wickham
Zachary

20. Fully indexed files The file is unordered
To access a file you search the index sequentially
Followed by direct access to the record
Harris
Shah
Oworu
Tooey
Wickham
Hussein
Heim
Hansen
Schmidt
Arnold
O’Hanlon
Zachary
Christopher
INDEX

21. Five-Minute Task
In groups, think carefully about exactly what you do when you look up someone’s name in the phone book.
Which access methods do you use?
What real-world objects correspond to:
The key
The record

22. Five-Minute Task Feedback
You open the book somewhere in the middle: Direct Access
You skip a few pages till you find the right one and then run your finger down the list to find the right name: Sequential Access
These two add up to: Indexed Sequential Access
The key must be the surname
The record could either be the address and phone number or, if you consider the address as a pointer, the house itself!

23. Review So far we have looked at: Sequential Access (unordered)
Sequential Access (ordered)
Direct Access (using hashing)
Direct Access (using fixed-length records)
Indexed Sequential Access

24. Five-minute task: Fill in the table
Access starts
Can access records in sequence?
Example
Sequential (unordered)
Sequential
(ordered)
Direct Access (hashing)
Direct Access (fixed-length)
Indexed Sequential

25. Five-minute task feedback: File Organisation Summary
Access starts
Can access records in sequence?
Example
Sequential (unordered)
Beginning of file 
HTTP log
Sequential (ordered) 
Video tape
Direct Access (hashing)
Anywhere
Booking system
Direct Access
(fixed length)
if the file is ordered
Account transactions
Indexed Sequential
Relational database table

26. Past Exam Question Higher Level Paper 2 May 2007

27. Past Exam Question Higher Level Paper 2 November 2008

28. Past Exam Question Higher Level Paper 2 May 2009