Re: Read Uncommitted

On Wed, 18 Dec 2019 at 14:06, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > I present a patch to allow READ UNCOMMITTED that is simple, useful and
> > efficient.
>
> Won't this break entirely the moment you try to read a tuple containing
> toasted-out-of-line values? There's no guarantee that the toast-table
> entries haven't been vacuumed away.
>
> I suspect it can also be broken by cases involving, eg, dropped columns.
> There are a lot of assumptions in the system that no one will ever try
> to read dead tuples.
>

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.

So this won't happen with the proposed patch.

> The fact that you can construct a use-case in which it's good for
> something doesn't make it safe in general :-(
>

I agree that safety is a concern, but I don't see any safety issues in the
patch as proposed.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Solutions for the Enterprise