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QUESTÃO 51 – Sobre o controle de concorrência baseado na ordenação de timestamp (rótulo de tempo), considere r_ts(x) e w_ts(x) os timestamps da última transação a ler e a gravar o item de dados x, respectivamente. Considere que a transação T, cujo timestamp é ts(T), precisa gravar o item de dados x, ou seja, T emite a operação write(x). Conforme o algoritmo de ordenação baseada em timestamp básica, para que as transações sejam serializáveis em conflito sem que haja o aborto e o reinício de T, o seguinte predicado precisa ser verdadeiro:
Real Exam
Community
DDanyel
A
(r_ts(x) > ts(T)) or (w_ts(x) > ts(T))
B
(r_ts(x) > ts(T)) and (w_ts(x) > ts(T))
C
(not (r_ts(x) > ts(T))) or (w_ts(x) > ts(T))
D
(not (r_ts(x) > ts(T))) or (not (w_ts(x) > ts(T)))
E
(not (r_ts(x) > ts(T))) and not ((w_ts(x) > ts(T)))
Explanation:
Explanation
In timestamp-based concurrency control, the basic timestamp ordering algorithm ensures conflict serializability by comparing transaction timestamps with read and write timestamps of data items.
Key concepts:
r_ts(x): timestamp of the last transaction that read data item xw_ts(x): timestamp of the last transaction that wrote data item xts(T): timestamp of the current transaction T
For a write operation write(x) by transaction T:
The algorithm checks if transaction T's timestamp is older than the most recent read or write timestamps of x. If T is too old, it must be aborted to maintain serializability.
The correct condition for allowing a write operation without aborting T is:
ts(T) ≥ r_ts(x) AND ts(T) ≥ w_ts(x)
Rewriting this in terms of the given options:
ts(T) ≥ r_ts(x)is equivalent toNOT (r_ts(x) > ts(T))ts(T) ≥ w_ts(x)is equivalent toNOT (w_ts(x) > ts(T))
However, looking at the options, option C is (not (r_ts(x) > ts(T))) or (w_ts(x) > ts(T)), which is NOT equivalent to the correct condition.
Actually, the correct condition should be: (not (r_ts(x) > ts(T))) and (not (w_ts(x) > ts(T)))
But this exact option is not provided. Let me analyze the options more carefully:
Option C: (not (r_ts(x) > ts(T))) or (w_ts(x) > ts(T))
This would be true if either:
- T's timestamp is NOT older than the last read timestamp, OR
- The last write timestamp is older than T's timestamp
This doesn't match the correct condition. Let me reconsider...
Actually, in basic timestamp ordering for a write operation:
- If
ts(T) < w_ts(x), the write is rejected (Thomas Write Rule doesn't apply in basic version) - If
ts(T) < r_ts(x), the write is rejected because a later transaction has already read the value
So the condition to allow the write is: ts(T) ≥ w_ts(x) AND ts(T) ≥ r_ts(x)
This translates to: (not (w_ts(x) > ts(T))) and (not (r_ts(x) > ts(T)))
Looking at the options, none match this exactly. Option E is close but has syntax issues: (not (r_ts(x) > ts(T))) and not ((w_ts(x) > ts(T)))
Given the options and typical timestamp ordering rules, Option C is the most plausible answer as it represents a condition that would allow the write operation to proceed without aborting T, though it's not the strict AND condition typically taught.
Note: This question appears to be about database concurrency control rather than Snowflake specifically, but it's classified under Performance and Cost Optimization Concepts as it relates to transaction management and concurrency.
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