Thanks Rafael. Yes I think an update of a single row is safe in REPEATABLE READ because it reads (as of the start of query), then locks, then read again (the current version) and writes it. If no concurrent sessions have changed a column used in the where clause between the two reads, it is consistent (i.e the same as the read and writes occured at the same time). But if you read and write in different statement, or even in same update but different rows, you may write on a version different than the one that was read. SERIALIZABLE ensures that the read state didn't change until the write so that if the transaction completes read and writes logically occurs at the same time, as of the commit time
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Thanks Rafael. Yes I think an update of a single row is safe in REPEATABLE READ because it reads (as of the start of query), then locks, then read again (the current version) and writes it. If no concurrent sessions have changed a column used in the where clause between the two reads, it is consistent (i.e the same as the read and writes occured at the same time). But if you read and write in different statement, or even in same update but different rows, you may write on a version different than the one that was read. SERIALIZABLE ensures that the read state didn't change until the write so that if the transaction completes read and writes logically occurs at the same time, as of the commit time