Re: cheaper snapshots

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Jul 28, 2011 at 10:33 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> But should we rethink that? Your point that hot standby transactions on
>> a slave could see snapshots that were impossible on the parent was
>> disturbing. Should we look for a way to tie "transaction becomes
>> visible" to its creation of a commit WAL record? I think the fact that
>> they are not an indivisible operation is an implementation artifact, and
>> not a particularly nice one.

> Well, I agree with you that it isn't especially nice, but it seems
> like a fairly intractable problem. Currently, the standby has no way
> of knowing in what order the transactions became visible on the
> master.

Right, but if the visibility order were *defined* as the order in which
commit records appear in WAL, that problem neatly goes away. It's only
because we have the implementation artifact that "set my xid to 0 in the
ProcArray" is decoupled from inserting the commit record that there's
any difference.

regards, tom lane