Simon Riggs <simon@xxxxxxxxxxxxxxx> wrote: > On 17 June 2015 at 13:52, Kevin Grittner <kgrittn@xxxxxxxxx> wrote: >> Filipe Pina <filipe.pina@xxxxxxxxxxxxx> wrote: >>> if drop the foreign key constraint on stuff_ext table there are >>> no failures at all… >> >> It is my recollection that we were excluding the queries used to >> enforce referential integrity constraints from the conflict >> tracking, so I am surprised you are seeing this. What is the exact >> version you are using (as reported by the version() function)? > > I don't see any mechanism for excluding anything from > serializable checks, so I can't see how that would work. It is a matter of where calls to PredicateLockXxx and CheckForSerializableConflictXxx calls were inserted into, for example, heap and index AM code. At least I think we omitted placing some at locations which were known to be used for RI enforcement; but apparently some more generic code is exercised by the RI trigger execution which can still trigger serialization failures based on FKs. > I can't find any mention of serializability concerns in the RI > code itself. It is mentioned in the README-SSI file. > AFAIK it would be strange to exclude FK checks from > serializability checks, since they represent a valid observation > of an intermediate state. The idea that this is OK is based on the observations in the paper "Automating the Detection of Snapshot Isolation Anomalies" by Sudhir Jorwekar, Alan Fekete, Krithi Ramamritham, and S. Sudarshan[1]. To quote a key sentence from that paper: | The database system ensures the preservation of some integrity | constraints which are explicitly declared to the system in the | schema definition, such as uniqueness of primary key and | referential integrity. Some of the SI anomalies are avoided due | to the dbms enforcement of these constraints. For all the gory details, please reference that paper. It was discussed during PostgreSQL SSI development, although I'm not sure whether that made it to the lists or was off-list discussion amongst those working on it at the time. Basically, the anomalies are avoided due to the enforcement of the constraints, and it is next to impossible to generate a serialization failure instead of the constraint failure due to the timings involved. Since the constraints are more narrowly targeted (always at the tuple level), using SSI techniques would be redundant effort (hurting performance) that could only generate false positives. If you see some exception to that which we missed, let's discuss. Perhaps that paper should be cited in the source code and/or README. > Mat Views are excluded but I don't understand why that should be > the case. There is no documented explanation. Good point; it should be documented. Basically, since the matview is a materialized copy of data from other relations from some prior point in time, the race conditions caught by SSI would be trivial compared to those likely to exist based on the elapsed time since the last REFRESH; so it would be kind of silly to try to enforce the more subtle interactions while ignoring the big, glaring, obvious one. It would be a bit like treating a laceration of someone's hand when they were not breathing -- it's not the thing to worry about. As we enhance matviews to have associated freshness information and especially once we use them like indexes to optimize queries this will deserve a close look, as there is likely to be something meaningful we can do at that time. Anyway, it appears that there is more that could be done to avoid generating serialization failures based on the actions of RI enforcement triggers. I don't think we can properly call it a bug, since it doesn't allow an incorrect state to be created; but it is clearly an opportunity for performance enhancement of the feature. One of many, unfortunately. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company [1] http://www.vldb.org/conf/2007/papers/industrial/p1263-jorwekar.pdf -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
