Let S be a stack of size n >= 1. Starting with the empty stack, suppose we push the first n natural numbers in sequence, and then perform n pop operations. Assume that Push and Pop operations take X seconds each, and Y seconds elapse between the end of one such stack operation and the start of the next operation. For m >= 1, define the stack-life of m as the time elapsed from the end of Push(m) to the start of the pop operation that removes m from S. The average stack-life of an element of this stack is

Consider three data items D1, D2, and D3, and the following execution schedule of transactions T1, T2, and T3. In the diagram, R(D) and W(D) denote the actions reading and writing the data item D respectively.

Which of the following statements is correct?

Which of the following scenarios may lead to an irrecoverable error in a database system?

Consider the following SQL query:

Select distinct a1, a2, ..., an 

From r1, r2, ..., rm 

Where P; 

For an arbitrary predicate P, this query is equivalent to which of the following relational algebra expressions?