Re: Read Uncommitted

Hi,

On 2019-12-18 18:06:21 +0000, Simon Riggs wrote:
> On Wed, 18 Dec 2019 at 17:35, Robert Haas  wrote:
> 
> > On Wed, Dec 18, 2019 at 10:18 AM Simon Riggs 
> > wrote:
> > > This was my first concern when I thought about it, but I realised that
> > by taking a snapshot and then calculating xmin normally, this problem would
> > go away.
> >
> > Why? As soon as a transaction aborts...
> >
> 
> So this is the same discussion as elsewhere about potentially aborted
> transactions...
> AFAIK, the worst that happens in that case is that the reading transaction
> will end with an ERROR, similar to a serializable error.

I don't think that's all that can happen. E.g. the toast identifier
might have been reused, and there might be a different datum in
there. Which then means we'll end up calling operators on data that's
potentially for a different datatype - it's trivial to cause crashes
that way. And, albeit harder, possible to do more than that.

I think there's plenty other problems too, not just toast. There's
e.g. some parts of the system that access catalogs using a normal
snapshot - which might not actually be consistent, because we have
various places where we have to increment the command counter multiple
times as part of a larger catalog manipulation.

Greetings,

Andres Freund