Determine Decomposition Is Lossless Or Lossy

Decomposition in DBMS-

 

Before you go through this article, make sure that you have gone through the previous article on Decomposition in DBMS.

 

We have discussed-

  • Decomposition is a process of dividing a single relation into two or more sub relations.
  • Decomposition of a relation can be completed in the following two ways-

 

 

  1. Lossless Join Decomposition
  2. Lossy Join Decomposition

 

In this article, we will learn how to determine whether the decomposition is lossless or lossy.

 

Determining Whether Decomposition Is Lossless Or Lossy-

 

Consider a relation R is decomposed into two sub relations R1 and R2.

Then,

  • If all the following conditions satisfy, then the decomposition is lossless.
  • If any of these conditions fail, then the decomposition is lossy.

 

Condition-01:

 

Union of both the sub relations must contain all the attributes that are present in the original relation R.

Thus,

R1 ∪ R2 = R

 

Condition-02:

 

  • Intersection of both the sub relations must not be null.
  • In other words, there must be some common attribute which is present in both the sub relations.

Thus,

R1 ∩ R2 ≠ ∅

 

Condition-03:

 

Intersection of both the sub relations must be a super key of either R1 or R2 or both.

Thus,

R1 ∩ R2 = Super key of R1 or R2

 

PRACTICE PROBLEMS BASED ON DETERMINING WHETHER DECOMPOSITION IS LOSSLESS OR LOSSY-

 

Problem-01:

 

Consider a relation schema R ( A , B , C , D ) with the functional dependencies A → B and C → D. Determine whether the decomposition of R into R1 ( A , B ) and R2 ( C , D ) is lossless or lossy.

 

Solution-

 

To determine whether the decomposition is lossless or lossy,

  • We will check all the conditions one by one.
  • If any of the conditions fail, then the decomposition is lossy otherwise lossless.

 

Condition-01:

 

According to condition-01, union of both the sub relations must contain all the attributes of relation R.

So, we have-

 R1 ( A , B ) ∪ R2 ( C , D )

= R ( A , B , C , D )

Clearly, union of the sub relations contain all the attributes of relation R.

Thus, condition-01 satisfies.

 

Condition-02:

 

According to condition-02, intersection of both the sub relations must not be null.

So, we have-

R1 ( A , B ) ∩ R2 ( C , D )

= Φ

Clearly, intersection of the sub relations is null.

So, condition-02 fails.

Thus, we conclude that the decomposition is lossy.

 

Problem-02:

 

Consider a relation schema R ( A , B , C , D ) with the following functional dependencies-

A → B

B → C

C → D

D → B

Determine whether the decomposition of R into R( A , B ) , R2 ( B , C ) and R3 ( B , D ) is lossless or lossy.

 

Solution-

 

Strategy to Solve

 

When a given relation is decomposed into more than two sub relations, then-

  • Consider any one possible ways in which the relation might have been decomposed into those sub relations.
  • First, divide the given relation into two sub relations.
  • Then, divide the sub relations according to the sub relations given in the question.

As a thumb rule, remember-

Any relation can be decomposed only into two sub relations at a time.

 

Consider the original relation R was decomposed into the given sub relations as shown-

 

 

Decomposition of R(A, B, C, D) into R'(A, B, C) and R3(B, D)-

 

To determine whether the decomposition is lossless or lossy,

  • We will check all the conditions one by one.
  • If any of the conditions fail, then the decomposition is lossy otherwise lossless.

 

Condition-01:

 

According to condition-01, union of both the sub relations must contain all the attributes of relation R.

So, we have-

 R ( A , B , C ) ∪ R3 ( B , D )

= R ( A , B , C , D )

Clearly, union of the sub relations contain all the attributes of relation R.

Thus, condition-01 satisfies.

 

Condition-02:

 

According to condition-02, intersection of both the sub relations must not be null.

So, we have-

 R ( A , B , C ) ∩ R3 ( B , D )

= B

Clearly, intersection of the sub relations is not null.

Thus, condition-02 satisfies.

 

Condition-03:

 

According to condition-03, intersection of both the sub relations must be the super key of one of the two sub relations or both.

So, we have-

 R ( A , B , C ) ∩ R3 ( B , D )

= B

Now, the closure of attribute B is-

B+ = { B , C , D }

Now, we see-

  • Attribute ‘B’ can not determine attribute ‘A’ of sub relation R’.
  • Thus, it is not a super key of the sub relation R’.
  • Attribute ‘B’ can determine all the attributes of sub relation R3.
  • Thus, it is a super key of the sub relation R3.

 

Clearly, intersection of the sub relations is a super key of one of the sub relations.

So, condition-03 satisfies.

Thus, we conclude that the decomposition is lossless.

 

Decomposition of R'(A, B, C) into R1(A, B) and R2(B, C)-

 

To determine whether the decomposition is lossless or lossy,

  • We will check all the conditions one by one.
  • If any of the conditions fail, then the decomposition is lossy otherwise lossless.

 

Condition-01:

 

According to condition-01, union of both the sub relations must contain all the attributes of relation R’.

So, we have-

 R( A , B ) ∪ R2 ( B , C )

= R’ ( A , B , C )

Clearly, union of the sub relations contain all the attributes of relation R’.

Thus, condition-01 satisfies.

 

Condition-02:

 

According to condition-02, intersection of both the sub relations must not be null.

So, we have-

 R1 ( A , B ) ∩ R2 ( B , C )

= B

Clearly, intersection of the sub relations is not null.

Thus, condition-02 satisfies.

 

Condition-03:

 

According to condition-03, intersection of both the sub relations must be the super key of one of the two sub relations or both.

So, we have-

 R1 ( A , B ) ∩ R2 ( B , C )

= B

Now, the closure of attribute B is-

B+ = { B , C , D }

Now, we see-

  • Attribute ‘B’ can not determine attribute ‘A’ of sub relation R1.
  • Thus, it is not a super key of the sub relation R1.
  • Attribute ‘B’ can determine all the attributes of sub relation R2.
  • Thus, it is a super key of the sub relation R2.

 

Clearly, intersection of the sub relations is a super key of one of the sub relations.

So, condition-03 satisfies.

Thus, we conclude that the decomposition is lossless.

 

Conclusion-

Overall decomposition of relation R into sub relations R1, R2 and R3 is lossless.

 

Next Article- Introduction to Normal Forms

 

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Summary
Determine Decomposition Is Lossless Or Lossy
Article Name
Determine Decomposition Is Lossless Or Lossy
Description
To determine whether decomposition is lossless or lossy, we check 3 conditions. Lossless Decomposition occurs if all conditions satisfy. Lossy decomposition occurs if any one condition fails.
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Gate Vidyalay
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