Re: SSI implementation question

Dan Ports <drkp@csail.mit.edu> writes:
> On Wed, Oct 19, 2011 at 04:36:41PM -0400, Tom Lane wrote:
>> (2) as things stand, xact A need not be running in serializable mode ---
>> if B is serializable, does *that* break any assumptions?

> [taking these in opposite order]

> Yes, I think that's going to be a problem. The obvious case where it's
> clearly not going to work is if A is older than the oldest active
> serializable xact (i.e. SxactGlobalXmin would have to move backwards).
> It's probably possible to make it work when that's not the case, but I
> think it's better to require A to be serializable -- if nothing else,
> it's a far simpler rule to document!

> There is another case that could be problematic, if A was READ ONLY,
> and B isn't. It sounds to me like that would also be a reasonable thing
> to forbid.

I've committed the synchronized-snapshots patch with those two
restrictions, ie, to import a snapshot into a serializable transaction
(1) the source transaction must be serializable (and must still be
running, of course);
(2) you can't import a read-only transaction's snapshot into a
read-write serializable transaction.

I don't understand the SSI code well enough to tell if this is
sufficient or not, so I hope you guys will take a closer look at the
issue when you have time.
        regards, tom lane