Give examples of schedules that would be accepted at
a. SNAPSHOT isolation but not REPEATABLE READ
b. SERIALIZABLE but not SNAPSHOT isolation
a. r1(x) r2(x) w2(x) commit2 w1(y) commit1
At REPEATABLE READ, T2 would not be allowed to write x since T1 would have a
read lock on it.
b. r1(x) r2(y) w1(x) commit1 r1(x) w2(x) commit2
This schedule is serializable, but would not be allowed at SNAPSHOT isolation. At
SNAPSHOT, T2’s read of x would give the value before T1 wrote it, and then T2
would not be allowed to commit because T1 was the first committer that wrote x.
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