A database system uses Write-Ahead Logging (WAL) to ensure atomicity and durability. Consider the following timeline for a single transaction T5:
1. ⟨T5 start⟩
2. ⟨T5, X, 200, 250⟩ — log record written and flushed
3. ⟨T5, Y, 300, 350⟩ — log record written and flushed
4. System crashes

Which statement best describes the state of data items X and Y on disk after recovery?

Question

A database system uses Write-Ahead Logging (WAL) to ensure atomicity and durability. Consider the following timeline for a single transaction T5: 
 1. ⟨T5 start⟩ 
 2. ⟨T5, X, 200, 250⟩ — log record written and flushed 
 3. ⟨T5, Y, 300, 350⟩ — log record written and flushed 
 4. System crashes  
 
 Which statement best describes the state of data items X and Y on disk after recovery?

Accepted Answer

Neither X nor Y reflects the new values, because T5 did not commit before the crash.

Answer

Both X and Y reflect the new values (250 and 350), because WAL ensures durability once the log records are flushed.

Answer

X reflects the new value (250) but Y does not, because the X log was written first.

Answer

Y reflects the new value (350) but X does not, because the Y log was more recent.

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