Re: [WIP] The relminxid addition, try 3

Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> 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.

You can't guarantee that. Consider for instance manual updates to
pg_class:

BEGIN;
UPDATE pg_class SET reltriggers = 0 WHERE relname = ...
... alter table contents ...
COMMIT or ROLLBACK;

I believe there are actually patterns like this in some pg_dump output.
Will you hack every UPDATE operation to test whether it's changing
pg_class and if so force an "unfreeze" operation before changing any
row? No thanks :-(

>> 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.

(Yawn...) We've made far bigger changes than that. The important
thing is to get it right.

regards, tom lane