Module 8 Final Quiz: Concurrency Control & Locking MCQs

1

What is a Dirty Read anomaly?

A. Reading data from a corrupted disk drive

B. Transaction A modifies a row without committing; Transaction B reads that uncommitted modified row; Transaction A then ABORTS (rolls back)

C. Reading a row that has a NULL value

D. Reading data over an unencrypted WiFi connection

2

What is a Non-Repeatable Read (Fuzzy Read) anomaly?

A. A query that returns an error when executed twice

B. Transaction A reads a row; Transaction B modifies or deletes that row and COMMITS; Transaction A reads the row again and observes a different value

C. A transaction inserting duplicate rows

D. A server running out of RAM buffer space

3

What is a Phantom Read anomaly?

A. Reading data deleted 10 years ago

B. Transaction A executes a range query (e.g. salary > 50k); Transaction B INSERTS a new row satisfying that predicate and COMMITS; Transaction A re-runs the range query and sees new 'phantom' rows

C. A crash caused by ghost memory pointers

D. A read that takes 0 milliseconds

4

Which ANSI SQL isolation level prevents Dirty Reads and Non-Repeatable Reads, but may still permit Phantom Reads?

A. Read Uncommitted

B. Read Committed

C. Repeatable Read

D. Serializable

5

Which SQL isolation level guarantees complete freedom from all concurrency anomalies by making execution equivalent to serial execution?

A. Read Committed

B. Serializable

C. Repeatable Read

D. Snapshot Isolation

6

What are the rules of Shared (S) and Exclusive (X) lock compatibility?

A. S locks conflict with S locks; X locks do not

B. Multiple transactions can concurrently hold Shared (S) locks on an item; but an Exclusive (X) lock conflicts with both S and X locks (strictly 1 holder)

C. X locks can only be held by the DBA

D. S locks can only be used on primary keys

7

What is the defining rule of the Two-Phase Locking (2PL) protocol?

A. Transactions must execute in two separate database servers

B. A transaction executes in two distinct phases: a Growing Phase (where locks are acquired and NO lock can be released) and a Shrinking Phase (where locks are released and NO lock can be acquired)

C. Every transaction must be committed twice

D. Locks are held for exactly 2 seconds

8

What is the difference between Strict 2PL and Rigorous 2PL?

A. Strict 2PL only locks tables; Rigorous 2PL locks columns

B. Strict 2PL holds all Exclusive (X) locks until transaction termination (COMMIT/ABORT); Rigorous 2PL holds ALL locks (both S and X) until transaction termination

C. Rigorous 2PL does not prevent cascading aborts; Strict 2PL does

D. Strict 2PL does not use locks

9

In the Wait-Die deadlock prevention scheme (where older transactions have smaller timestamps), what happens if an Older transaction requests a resource held by a Younger transaction?

A. The older transaction dies

B. The older transaction is allowed to WAIT ($TS(T_{old}) < TS(T_{young})$)

C. The younger transaction is killed immediately

D. Both transactions commit

10

In the Wound-Wait deadlock prevention scheme, what happens if an Older transaction requests a resource held by a Younger transaction?

A. The older transaction waits

B. The older transaction 'wounds' (preempts/aborts) the younger transaction, taking the resource immediately ($TS(T_{old}) < TS(T_{young})$)

C. Both transactions are suspended

D. The younger transaction ignores the request

11

How does Thomas' Write Rule modify the standard Timestamp Ordering protocol?

A. It requires all writes to be timestamped with UTC time

B. If transaction $T_i$ attempts to write data item $Q$ but its timestamp is older than $Q$'s write timestamp ($TS(T_i) < W\text{-}timestamp(Q)$), the write is simply IGNORED rather than aborting $T_i$

C. It eliminates read timestamps completely

D. It forces transactions to write in reverse chronological order

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