Consider concurrent execution of two transactions $T_1$ and $T_2$ in a DBMS, both of which access a data object $A$. For these two transactions to not conflict on $A$, which one of the following statements must be true?

Question

A database has 25,000 fixed-length records of 100 bytes (primary key 15 bytes). The data file is block-aligned so each record is fully contained within a block. The file is indexed by a primary index file, also block-aligned and ordered. Block size is 1024 bytes and a block pointer is 5 bytes. Each index entry stores the block’s anchor key (15 bytes) and a pointer (5 bytes). Binary search on an index file of $b$ blocks needs $\lceil \log_2 b \rceil$ block accesses in the worst case. Given a key, the number of block accesses required to identify the data file block that may contain the record, in the worst case, is ____________.

Answer 1

This question is about when two transactions touching the same data item A can be considered non-conflicting. In concurrency control, an operation pair conflicts only when the two operations belong to different transactions, act on the same item, and at least one of them is a write, because only then does the execution order affect the result.

Both $T_1$ and $T_2$ only read $A$: reading never changes the value stored in A, so however the two reads are interleaved, both transactions see the same value and the database state after both operations is identical either way. There is no dependency on order, so this pair does not conflict.
$T_1$ reads $A$ and $T_2$ writes $A$: whether $T_1$'s read happens before or after $T_2$'s write changes what value $T_1$ actually sees, the old value or the new one. Since the outcome depends on order, this is a conflicting read-write pair.
$T_1$ writes $A$ and $T_2$ reads $A$: this is the same situation with the roles reversed; $T_2$'s read result depends on whether it runs before or after $T_1$'s write, so it also conflicts.
Both $T_1$ and $T_2$ write $A$: the value left in A at the end depends on which write executes last, so swapping the order of the two writes changes the final state of the database. This is a conflicting write-write pair.

Only the read-read case is free of conflict, because it is the only pairing where neither operation can alter what the other one observes or leaves behind.

Let's summarize:

A conflict needs at least one write among the two operations; a pure read-read pair can never conflict.
Read-write, write-read and write-write are all conflicting because their outcome changes with execution order.

So the statement that must be true for $T_1$ and $T_2$ to not conflict on A is that both only read A, option (A).

Answer 2

This question is about when two transactions touching the same data item A can be considered non-conflicting. In concurrency control, an operation pair conflicts only when the two operations belong to different transactions, act on the same item, and at least one of them is a write, because only then does the execution order affect the result.

Both $T_1$ and $T_2$ only read $A$: reading never changes the value stored in A, so however the two reads are interleaved, both transactions see the same value and the database state after both operations is identical either way. There is no dependency on order, so this pair does not conflict.
$T_1$ reads $A$ and $T_2$ writes $A$: whether $T_1$'s read happens before or after $T_2$'s write changes what value $T_1$ actually sees, the old value or the new one. Since the outcome depends on order, this is a conflicting read-write pair.
$T_1$ writes $A$ and $T_2$ reads $A$: this is the same situation with the roles reversed; $T_2$'s read result depends on whether it runs before or after $T_1$'s write, so it also conflicts.
Both $T_1$ and $T_2$ write $A$: the value left in A at the end depends on which write executes last, so swapping the order of the two writes changes the final state of the database. This is a conflicting write-write pair.

Only the read-read case is free of conflict, because it is the only pairing where neither operation can alter what the other one observes or leaves behind.

Let's summarize:

A conflict needs at least one write among the two operations; a pure read-read pair can never conflict.
Read-write, write-read and write-write are all conflicting because their outcome changes with execution order.

So the statement that must be true for $T_1$ and $T_2$ to not conflict on A is that both only read A, option (A).

Consider concurrent execution of two transactions \(T_1\) and \(T_2\) in a DBMS, both of which access a data object \(A\). For these two transactions to not conflict on \(A\), which one of the following statements must be true?

Show Hint

The Correct Option is A

Solution and Explanation

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