GATE Databases 3

Question 1

In a hashed file organization, if the bucket size is B and there are N records, the expected number of disk I/Os for a search with no overflow is:

Accepted Answer

O(1)

Answer

With a good hash function and no overflow, hashing directly computes the bucket address in O(1) disk I/Os.

Question 2

Which of the following SQL constraints ensures that a column value in one table exists in another table?

Accepted Answer

FOREIGN KEY

Answer

A FOREIGN KEY constraint enforces referential integrity by requiring the value to exist as a primary key in the referenced table.

Question 3

The two-phase locking (2PL) protocol guarantees:

Accepted Answer

Serializability of transactions

Answer

2PL guarantees conflict-serializability by requiring all lock acquisitions to precede any lock release.

Question 4

Which functional dependency set is equivalent to {A→BC, B→C, AB→C}?

Accepted Answer

{A→BC, B→C}

Answer

AB→C is redundant since A→BC already implies A→C, so the minimal equivalent set is {A→BC, B→C}.

Question 5

What does the term 'phantom read' refer to in transaction isolation?

Accepted Answer

A query returning rows that were inserted by another transaction mid-execution

Answer

A phantom read occurs when a transaction re-executes a range query and finds new rows inserted by another committed transaction.

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