On 15.06.2011 23:28, Dan Ports wrote:
+SSI is based on the observation [2] that each snapshot isolation +anomaly corresponds to a cycle that contains a "dangerous structure" +of two adjacent rw-conflict edges: + + Tin ------> Tpivot ------> Tout + rw rw + +SSI works by watching for this dangerous structure, and rolling +back a transaction when needed to prevent any anomaly. This means it +only needs to track rw-conflicts between concurrent transactions, not +wr- and ww-dependencies. It also means there is a risk of false +positives, because not every dangerous structure corresponds to an +actual serialization failure. + +The PostgreSQL implementation uses two additional optimizations: + +* Tout must commit before any other transaction in the cycle + (see proof of Theorem 2.1 of [2]). We only roll back a transaction + if Tout commits before Tpivot and Tin. + +* if Tin is read-only, there can only be an anomaly if Tout committed + before Tin takes its snapshot. This optimization is an original + one. Proof: + + - Because there is a cycle, there must be some transaction T0 that + precedes Tin in the serial order. (T0 might be the same as Tout). + + - The dependency between T0 and Tin can't be a rw-conflict, + because T1 was read-only, so it must be a ww- or wr-dependency. + Those can only occur if T0 committed before T1 started.
There's no mention on what T1 is. I believe it's supposed to be Tin, in the terminology used in the graph.
I don't see how there can be a ww-dependency between T0 and Tin. There can't be a rw-conflict because Tin is read-only, so surely there can't be a ww-conflict either?
(the proof is still valid, though)
+ - Because Tout must commit before any other transaction in the + cycle, it must commit before T0 commits -- and thus before T1 + starts.
Another reference to T1. Patch looks good otherwise. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
