• Lossless join decomposition:
  • The decomposition of relation R into R1 and R2 is lossless when the join of R1 and R2 yields the same relation as in R.
  • This is also referred as non-additive decomposition.
  • Example:
  • The same relation Account figure (a) is decomposed into two relation- Acc_Bal and Acc_Branch – as shown in figure (b).
  • Now, when these two relations are joined on the common column ‘ano’, the resultant relation will look like Acc_Joined relations as shown figure (c). This relation is exactly same as that of original relation Account.

Account:

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  1.  
  1.  
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  1.  
  1.  
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Figure (a): An Account relation

Acc_Bal:                        Acc_Branch:

Ano

Balance

A01

5000

A02

5000

balance

Bname

5000

Vvn

5000

Ksad

Figure (b): Decomposed relation- Acc_Bal and Acc_Branch relation

Acc_Joined:   

Ano

Balance

Bname

A01

5000

Vvn

A02

5000

Ksad

Figure(c): Joined relation – Acc_Joined relation

  • In lossless decomposition, no any spurious tuples are generated when a natural join is applied to the relations in the decomposition.
  • In other words, decomposition is lossy if R= join of R1 and R2, over X, where R is join is applied to the relations in decomposition.