Re: [WIP] The relminxid addition, try 3

Tom Lane wrote:
> I wrote:
> > Well, that needs rethinking.  The unfreeze has to be a non-transactional
> > update (if our transaction rolls back, the unfreeze still has to
> > "stick", because we may have put dead tuples into the rel).
>
> Actually, this seems even messier than I thought.  Consider a
> transaction that does something transactional to a table's schema,
> thereby generating a new pg_class row (but not touching any data within
> the table), and then alters the table contents, requiring an unfreeze.
> An update to the apparently-current pg_class tuple is not good because
> that tuple might be rolled back.  An update to the last committed
> version doesn't work either.

Well, if a transaction modifies a table in some way, even without
changing the data, should generate an unfreeze event, because it will
need to lock the table; for example AlterTable locks the affected
relation with AccessExclusiveLock.  It's important for the
non-transactional change to the pg_class tuple be the very first in the
transaction, because otherwise the change could be lost; but other than
this, I don't think there's any problem.

Not that I had actually considered this problem, to be frank; but it
seems a nice side effect of how the unfreezing works.

> This seems real close to the recent discussions about how to put
> sequence data into a single one-row-per-sequence system catalog,
> specifically about how there were some parts of the sequence catalog
> data that should be transactional and some that should not be.
> I'm wondering if we need a second pg_class-derived catalog that carries
> just the nontransactional columns.

I hope we don't need to do this because ISTM it will be a very big change.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.