Transcript S5

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.
1
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.
2
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).
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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.
4
Possible representation
Athens .
S1
Smith
London .
.
S2
Jones
S3
Black
Paris
S4
Clark
.
S5
Adams
Fig 5: 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
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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.
15
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.
16
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.
17
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
19
Possible representation
SNAME
Pointers
STATUS
Pointers
Smith
^S1
10
^S2
Jones
^S2
20
^S1, ^S4
Blake
^S3
30
^S3, ^S5
Clark
^S4
Adams
^S5
STATUS Index
SNAME Index
Fig 6: Inverted Organization
20
Possible representation
SNAME
Pointers
STATUS
Pointers
CITY
Pointers
Smith
^S1
10
^S2
Athens
^S5
Jones
^S2
20
^S1, ^S4
London
^S1, ^S4
Blake
^S3
30
^S3, ^S5
Paris
^S3, ^S2
Clark
^S4
Adams
^S5
STATUS Index
CITY Index
SNAME Index
Fig 6: Inverted Organization
21
Possible representation
SNAME
Pointers
CITY
Pointers
S#
Smith
^S1
Athens
^S5
S1
Jones
^S2
London
^S1, ^S4
S2
Blake
^S3
Clark
^S4
Adams
^S5
Paris
^S3, ^S2
CITY Index
S3
S4
S5
SUPPLIER FILE
SNAME Index
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
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