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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.

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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.

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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), a status value (STATUS), and location (CITY).

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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

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

..

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

..

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

..

Fig 5: Pointer chain

. .

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Possible representation

Athens

. London

Paris

S1

Smith

20

. S2

Jones 10

S3

Black

30 S4

Clark

20 S5

Adams

30

.

.

..

Fig 5: Pointer chain

. .

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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.

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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.

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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.

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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.

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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).

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Possible representation

SNAME Pointers

Smith ^S1

Jones ^S2

Blake ^S3

Clark ^S4

Adams ^S5

SNAME Index

Fig 6: Inverted Organization

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Possible representation

SNAME Pointers

Smith ^S1

Jones ^S2

Blake ^S3

Clark ^S4

Adams ^S5

STATUS

Pointers

10 ^S2

20 ^S1, ^S4

30 ^S3, ^S5

SNAME Index

STATUS Index

Fig 6: Inverted Organization

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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

SNAME Index

STATUS Index CITY Index

Fig 6: Inverted Organization

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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

S#

S1

S2

S3

S4

S5

SNAME Index

STATUS Index CITY Index

SUPPLIER

File

Fig 6: Inverted Organization

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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.

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Summary: In this learning sequence, we discussed the following topic:

- Three possible representations for some sample data.

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END