Question

If a relational decomposition is not dependency-preserving, which one of the following relational operators will be executed more frequently in order to maintain the dependencies?

• A. Selection
• B. Projection
• C. Join
• D. Set union

Hint:

When dealing with non-dependency-preserving decompositions in relational databases, join operations are essential to maintain the required functional dependencies, as they help in combining fragmented data that is needed for dependency enforcement.

Answer 1

The Correct Option is C

In relational database theory, a relational decomposition is dependency-preserving if, after decomposing a relation, we can enforce all the functional dependencies without needing to perform a join operation. If the decomposition is not dependency-preserving, we need to perform additional operations to maintain the dependencies.
The Join operator is the most commonly used operator when dependencies need to be maintained. This is because, in a non-dependency-preserving decomposition, we often have to join relations to reconstruct the information required to enforce the dependencies.
Selection and Projection only reduce the size of the relation, but they do not help in preserving or enforcing dependencies.
Set union is not typically used to enforce dependencies.
Therefore, Join is the operator that will be executed more frequently to maintain dependencies in the absence of dependency-preserving decomposition.