What is Correctness Rules of Fragmentation?

There Three Correctness Rules

Completeness

If relation R is decomposed into fragments R1,R2,……Rn .

Each data item that can be found in R must appear in at least one fragment

Reconstruction

Reconstruction: it should be possible to define a relational operator such that the original relation can be reconstructed

R = g(R₁, R₂, …, R_n)

Reconstruction ensures that the dependencies of the original relation are preserved in fragmentation

Must be possible to define a relational operation that will reconstruct R from the fragments.

Reconstruction for horizontal fragmentation is union operation and join for vertical

Disjoint ness

If data item d₁ appears in fragment R1 .

Then it should not appear in any other fragment

Exception Vertical fragmentation where primary key attributes must be repeated to allow reconstruction.

For horizontal fragmentation data item is a tuple

For vertical fragmentation data item is an attribute

Safdar Dogar