Re: logical changeset generation v6.2

On Tue, Oct 22, 2013 at 1:08 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> It seems to me that you have to think of the CTID map as tied to a
>> relfilenode; if you try to use one relfilenode's map with a different
>> relfilenode, it's obviously not going to work. So don't do that.
>
> It has to be tied to relfilenode (+ctid) *and* transaction
> unfortunately.

I agree that it does, but it doesn't seem particularly unfortunate to me.

>> That strikes me as a flaw in the implementation rather than the idea.
>> You're presupposing a patch where the necessary information is
>> available in WAL yet you don't make use of it at the proper time.
>
> The problem is that the mapping would be somewhere *ahead* from the
> transaction/WAL we're currently decoding. We'd need to read ahead till
> we find the correct one.

Yes, I think that's what you need to do.

> But I think I mainly misunderstood what you proposed. That mapping could
> be written besides relfilenode, instead of into the WAL. Then my
> imagined problem doesn't exist anymore.

That's pretty ugly. I think it should be written into WAL.

--
Robert Haas
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