Timestamp-based protocols in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the timestamp column from the transactions table.

DBMS Theory

SELECT [1] FROM transactions;

Drag options to blanks, or click blank then click option'

Astatus

Btransaction_id

Cuser_id

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting a column that does not store time information.

Confusing transaction ID with timestamp.

The timestamp column stores the time when each transaction occurred, which is essential for timestamp-based protocols.

2fill in blank

medium

Complete the code to retrieve transactions ordered by their timestamp in ascending order.

DBMS Theory

SELECT * FROM transactions ORDER BY [1] ASC;

Drag options to blanks, or click blank then click option'

Atimestamp

Bstatus

Cuser_id

Dtransaction_id

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by transaction ID instead of timestamp.

Using descending order when ascending is required.

Ordering by timestamp in ascending order lists transactions from oldest to newest, which is important in timestamp-based concurrency control.

3fill in blank

hard

Fix the error in the query to select transactions with timestamp greater than a given value.

DBMS Theory

SELECT * FROM transactions WHERE timestamp [1] '2024-01-01 00:00:00';

Drag options to blanks, or click blank then click option'

A<=

B>

C=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using = which selects only exact matches.

Using <= or < which select older transactions.

The operator > selects transactions with timestamps after the specified date, which is correct for filtering newer transactions.

4fill in blank

hard

Fill both blanks to create a query that selects transaction IDs and timestamps for transactions after a certain time.

DBMS Theory

SELECT [1], [2] FROM transactions WHERE timestamp > '2024-01-01 00:00:00';

Drag options to blanks, or click blank then click option'

Atransaction_id

Btimestamp

Cuser_id

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns unrelated to transaction identity or time.

Mixing up column names.

Selecting transaction_id and timestamp shows the unique ID and time of each transaction after the specified date.

5fill in blank

hard

Fill all three blanks to create a query that counts transactions per user after a certain timestamp.

DBMS Theory

SELECT [1], COUNT([2]) FROM transactions WHERE timestamp > '2024-01-01 00:00:00' GROUP BY [3];

Drag options to blanks, or click blank then click option'

Auser_id

B*

Dtransaction_id

Attempts:

3 left

💡 Hint

Common Mistakes

Counting a specific column instead of all transactions.

Grouping by the wrong column.

This query counts all transactions (*) grouped by each user_id for transactions after the given timestamp.

Practice

1. What is the main purpose of timestamps in timestamp-based protocols in DBMS?

easy

A. To give each transaction a unique order number

B. To encrypt the data during transactions

C. To store the transaction results permanently

D. To backup the database automatically

Solution

1. Step 1: Understand the role of timestamps

   Timestamps assign a unique number to each transaction to order them.

2. Step 2: Identify the main purpose in protocols

   This ordering helps the system decide which transaction can read or write data safely.

3. Final Answer:

   To give each transaction a unique order number -> Option A

4. Quick Check:

   Timestamps = Unique order number [OK]

Hint: Timestamps order transactions uniquely [OK]

Common Mistakes:

• Confusing timestamps with encryption keys
• Thinking timestamps store data permanently
• Assuming timestamps handle backups

2. Which of the following is the correct way to assign a timestamp to a transaction in a timestamp-based protocol?

easy

A. Assign the current system time as the timestamp

B. Assign a random number as the timestamp

C. Assign the transaction ID as the timestamp

D. Assign the timestamp after the transaction commits

Solution

1. Step 1: Understand timestamp assignment

   Timestamps are usually assigned based on the current system time to reflect order.

2. Step 2: Evaluate options

   Random numbers or transaction IDs do not guarantee correct ordering; timestamps must be assigned before commit.

3. Final Answer:

   Assign the current system time as the timestamp -> Option A

4. Quick Check:

   Timestamp = Current system time [OK]

Hint: Use system time for timestamps [OK]

Common Mistakes:

• Using random numbers instead of time
• Assigning timestamp after commit
• Confusing transaction ID with timestamp

3. Consider two transactions T1 and T2 with timestamps TS(T1) = 5 and TS(T2) = 10. If T1 tries to write a data item that T2 has already written, what will happen according to timestamp-based protocols?

medium

A. T1 will overwrite T2's write because it started earlier

B. T2 will be rolled back because it started later

C. T1 will be rolled back to prevent overwriting newer data

D. Both transactions will commit successfully

Solution

1. Step 1: Compare timestamps of T1 and T2

   T1 has timestamp 5 (older), T2 has timestamp 10 (newer).

2. Step 2: Apply timestamp protocol rule

   Older transactions cannot overwrite changes made by newer transactions, so T1 must be rolled back.

3. Final Answer:

   T1 will be rolled back to prevent overwriting newer data -> Option C

4. Quick Check:

   Older transaction rollback = T1 rollback [OK]

Hint: Older transactions cannot overwrite newer writes [OK]

Common Mistakes:

• Thinking older transactions can overwrite newer writes
• Assuming both transactions commit
• Confusing which transaction rolls back

4. Identify the error in this scenario: Transaction T3 with timestamp 15 reads a data item last written by T4 with timestamp 20. According to timestamp-based protocols, what is wrong if T3 reads the old value?

medium

A. No error, reading old value is allowed

B. T4 should roll back because T3 read first

C. T3 can read any value without restrictions

D. T3 should not read because it is older than T4's write

Solution

1. Step 1: Compare timestamps of T3 and T4

   T3 has timestamp 15 (older), T4 has timestamp 20 (newer).

2. Step 2: Apply read rule in timestamp protocol

   Older transactions cannot read data written by newer transactions because it causes inconsistency.

3. Final Answer:

   T3 should not read because it is older than T4's write -> Option D

4. Quick Check:

   Older transaction reading newer write = Not allowed [OK]

Hint: Older transactions cannot read newer writes [OK]

Common Mistakes:

• Assuming older transactions can read newer writes
• Thinking newer transaction rolls back
• Believing reading old value is always allowed

5. In a timestamp-based protocol, if a transaction T5 with timestamp 25 tries to write a data item that was last read by transaction T6 with timestamp 30, what should happen to maintain consistency?

hard

A. T6 should be rolled back because it read before T5's write

B. T5 should be rolled back because it is older than T6

C. Both transactions can proceed without rollback

D. T5 can write only if T6 commits first

Solution

1. Step 1: Analyze timestamps of T5 and T6

   T5 has timestamp 25 (older), T6 has timestamp 30 (newer).

2. Step 2: Apply write rule in timestamp protocol

   Older transaction T5 cannot write if a newer transaction T6 has already read the data, to avoid inconsistency.

3. Step 3: Decide action to maintain consistency

   T5 must be rolled back to prevent overwriting data read by newer transaction T6.

4. Final Answer:

   T5 should be rolled back because it is older than T6 -> Option B

5. Quick Check:

   Older write after newer read = Rollback older [OK]

Hint: Older writes after newer reads cause rollback [OK]

Common Mistakes:

• Allowing older writes after newer reads
• Rolling back newer transaction incorrectly
• Assuming both can proceed safely