Re: Exposing the Xact commit order to the user

2010/5/25 Florian Pflug <fgp(at)phlo(dot)org>:

> Hm, but for there to be an actual problem (and not a false positive), an
> actual dangerous circle has to exist in the dependency graph. The
> existence of a dangerous structure is just a necessary (but not
> sufficient) and easily checked-for condition for that, right? Now, if a
> read-only transaction only ever has outgoing edges, it cannot be part
> of a (dangerous or not) circle, and hence any dangerous structure it is
> part of is a false positive.
>
> I guess my line of reasoning is flawed somehow, but I cannot figure out why...

In the general case, "wr" dependencies also create "must be serialized
before" edges. It seems that those edges can be discarded when finding
a pivot, but if you want to go "back to basics":

("<" means "must be serialized before".)

* T1 < T2, because T1 reads a version of a data element for which T2
later creates a newer version (rw between T1 and T2).
* T3 < T1, because T3 reads a version of a data element for which T1
later creates a newer version (rw between T3 and T1).
* T2 < T3, because T2 creates a version of a data element, which is
then read by T3 (wr between T2 and T3).

(As you can see, those 3 edges form a cycle.)

Nicolas