On Tue, Nov 13, 2018 at 10:45 PM Kuntal Ghosh <kuntalghosh.2...@gmail.com> wrote:
> Currently, we're working on the serializable implementations for > zheap. Great! > If transaction T1 reads a row version (thus acquiring a predicate lock > on it) and a second transaction T2 updates that row version (thus > creating a rw-conflict graph edge from T1 to T2), must a third > transaction T3 which re-updates the new version of the row also have a > rw-conflict in from T1 to prevent anomalies? In other words, does it > matter whether we recognize the edge T1 --rw--> T3? No. Keep in mind that extensive study has shown that snapshot isolation can only be non-serializable if there is a cycle in the apparent order of execution and that this can only occur if there is a "dangerous structure" of two adjacent read-write antidependencies (a/k/a read-write dependencies, a/k/a rw-conflicts) *AND* the transaction you identify as T3 in that structure *IS THE FIRST TRANSACTION IN THE CYCLE TO COMMIT*. Looking at the implied T1/T3 relationship and looking for a T4 to complete the structure is not necessary, because there are proofs that three *ADJACENT* transactions are necessary for a serialization anomaly to be seen. -- Kevin Grittner VMware vCenter Server https://www.vmware.com/
