Re: serialization failure why?

Simon Riggs <simon@2ndQuadrant.com> wrote:
> On 17 June 2015 at 13:52, Kevin Grittner <kgrittn@ymail.com> wrote:
>> Filipe Pina <filipe.pina@impactzero.pt> 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:

> 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.