Unrepeatable reads: Assume T7 reads X twice during its implementation. If it did not update X itself it could be very disturbing to see a dissimilar value of X in its next read. But this could occur if, among the two read operations, another transaction modifies X.
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T7
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T8
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Assumed value of
X=2000
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T7
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T8
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READ X
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2000
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UPDATE X
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3000
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READ X
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3000
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Therefore, the inconsistent values are read and results of the transaction may be in fault.