Re: [PATCH 8/8] Introduce wal decoding via catalog timetravel

On Thursday, October 11, 2012 04:16:39 AM Greg Stark wrote:
> On Thu, Oct 11, 2012 at 2:40 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I think I've mentioned it before, but in the interest of not being
> >> seen to critique the bikeshed only after it's been painted: this
> >> design gives up something very important that exists in our current
> >> built-in replication solution, namely pipelining.
> >
> > Isn't there an even more serious problem, namely that this assumes
> > *all* transactions are serializable? What happens when they aren't?
> > Or even just that the effective commit order is not XID order?
>
> Firstly, I haven't read the code but I'm confident it doesn't make the
> elementary error of assuming commit order == xid order. I assume it's
> applying the reassembled transactions in commit order.
Yes, its commit order.

Imo commit order is more like assuming all transactions are done in read
committed and not above than assuming serializable? Or am I missing something?

> I don't think it assumes the transactions are serializable because
> it's only concerned with writes, not reads. So the transaction it's
> replaying may or may not have been able to view the data written by
> other transactions that commited earlier but it doesn't matter when
> trying to reproduce the effects using constants. The data written by
> this transaction is either written or not when the commit happens and
> it's all written or not at that time. Even in non-serializable mode
> updates take row locks and nobody can see the data or modify it until
> the transaction commits.
Yes. There will be problems if you want to make serializable work across
nodes, but that seems like something fiendishly complex anyway. I don't plan to
work on it in the forseeable future.

Greetings,

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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