Dirty Reads: T10 reads a value which is updated by T9. This update has not been committed and T9 aborts.
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T9
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T10
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Value of x old value =
200
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T9
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T10
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UPDATE X
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500
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READ X
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500
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ROLLBACK
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200
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?
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Here T10 reads a value that is updated by transaction T9 that has been aborted. Therefore T10 has read a value that would never exist in the database and hence the trouble. Here the difficulty is primarily of isolation of transaction.