Row cannot be located for updating some values may
standard defines four levels of transaction isolation.The most strict is Serializable, which is defined by the standard in a paragraph which says that any concurrent execution of a set of Serializable transactions is guaranteed to produce the same effect as running them one at a time in some order.(This is hardly surprising -- if the effect of the transactions must be consistent with having been run one at a time, how could you see any phenomena caused by interactions?) The phenomena which are prohibited at various levels are: A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.The reason that only provides three isolation levels is that this is the only sensible way to map the standard isolation levels to the multiversion concurrency control architecture.
But internally, there are only three distinct isolation levels, which correspond to the levels Read Committed, Repeatable Read, and Serializable.This behavior makes Read Committed mode unsuitable for commands that involve complex search conditions; however, it is just right for simpler cases.For example, consider updating bank balances with transactions like: If two such transactions concurrently try to change the balance of account 12345, we clearly want the second transaction to start with the updated version of the account's row.To set the transaction isolation level of a transaction, use the command SET TRANSACTION.in terms of searching for target rows: they will only find target rows that were committed as of the command start time.