1. “ Possible representation (2) “
Welcome: To the fifth learning sequence
“ Possible representation (2) “
Recap : In the previous learning sequence, we discussed four possible representations.
Present learning: We shall explore the following topics:
- The pointer chain.
- The chain two-way.
- The inverted organization.

2. Possible representation
- In this section we make a simple collection of sample data and consider some of many ways it could be represented in storage at the level of stored record interface.

3. Possible representation
The sample data is consists of information about the same five suppliers; for each supplier we wish to record the same attributes:
Supplier number (S#), a supplier name (SNAME), and location (CITY).

4. Possible representation
5- An alternative to the previous representation (Combination Factoring out the city and Indexing on city) that avoids this problem is illustrated in fig5. The pointer chain.

5. Possible representation
Athens .
London .
Paris . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams
Fig 5: Pointer chain

6. Possible representation
Athens .
London .
Paris . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams
Fig 5: Pointer chain

7. Possible representation
Athens .
London .
Paris . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

8. Possible representation
Athens .
London .
Paris . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

9. Possible representation
Athens .
London .
Paris . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

10. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

11. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

12. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

13. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 5: Pointer chain

14. Possible representation
In this representation each occurrence (CITY or supplier) contains just one pointer. Each City pointed to the first supplier in that city. That supplier then points to the second supplier record occurrence in the same city, who points to the third, and so on, up to the last, which points back to the city.

15. Possible representation
Thus for each city we have chain of all supplier in that city. The advantage of this representation is that it is easier to apply change.
The disadvantage is that, for a given city the only way to access the nth supplier is to follow the chain and access the 1st, 2nd,....,(n-1) the supplier too.
If each access involves a seek operation, the time taken to access the nth supplier may be quite considerable.

16. Possible representation
6- Another extension to the above representation might be making the chain two-way (so that stored record contains exactly two pointers).
The DBA might choose such a representation if deleting a supplier record is a common operation.

17. Possible representation
Fig5 (pointer chain) is a simple example of multilist organization when for each city we had a list of corresponding suppliers. In exactly the same way we could also have a list of suppliers for each distinct status value. In general a multilist organization can clearly contain any number of such lists.

18. Possible representation
7- To return to secondly indexing: just as it is possible to provide any number of lists in the multilist organization. It is also possible to provide any number of secondary indexes in organization.
In the extreme case we have the situation illustrated in fig6 an indexing on every secondary field, or called inverted organization (the symbol ^ is used to mean pointer to).

19. Possible representation
SNAME
Pointers
Smith
^S1
Jones
^S2
Blake
^S3
Clark
^S4
Adams
^S5
SNAME Index
Fig 6: Inverted Organization

20. Possible representation
SNAME
Pointers
Smith
^S1
Jones
^S2
Blake
^S3
Clark
^S4
Adams
^S5
STATUS
Pointers 10
^S2 20
^S1, ^S4 30
^S3, ^S5
STATUS Index
SNAME Index
Fig 6: Inverted Organization

21. Possible representation
SNAME
Pointers
Smith
^S1
Jones
^S2
Blake
^S3
Clark
^S4
Adams
^S5
STATUS
Pointers 10
^S2 20
^S1, ^S4 30
^S3, ^S5
CITY
Pointers
Athens
^S5
London
^S1, ^S4
Paris
^S3, ^S2
STATUS Index
CITY Index
SNAME Index
Fig 6: Inverted Organization

22. Possible representation
SNAME
Pointers
Smith
^S1
Jones
^S2
Blake
^S3
Clark
^S4
Adams
^S5 S# S1 S2 S3 S4 S5
CITY
Pointers
Athens
^S5
London
^S1, ^S4
Paris
^S3, ^S2
CITY Index
SUPPLIER FILE
SNAME Index
Fig 6: Inverted Organization

23. Possible representation
The organization will give good performance in response to a request for (all supplier with a given property).
A request for (all properties of given supplier) will take a long time to answer.

24. Summary: In this learning sequence, we discussed the following topic:
- Three possible representations for some sample data.

25. END