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

On 11 October 2012 03:16, Greg Stark <stark(at)mit(dot)edu> 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.
>
> 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.

This uses Commit Serializability, which is valid, as you say.

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