On Tue, Jan 11, 2011 at 12:37:28PM +0000, Simon Riggs wrote:
> On Tue, 2011-01-11 at 07:14 -0500, Noah Misch wrote:
>
> > These changes do make it harder to guess how much work the ALTER TABLE
> > will do. Indeed, about 1/4 of my own guesses prior to writing were
> > wrong. Something like WITHOUT REWRITE might be the way to go, though
> > there are more questions: if it does not rewrite, does it scan the
> > table? Which indexes, if any, does it rebuild? Which foreign key
> > constraints, if any, does it recheck? With patch 0, you can answer
> > all these questions by enabling DEBUG1 messages and trying the command
> > on your test system. For this reason, I did consider adding a VERBOSE
> > clause to show those messages at DETAIL, rather than unconditionally
> > showing them at DEBUG1. In any case, if a WITHOUT REWRITE like you
> > describe covers the important question, it's certainly easy enough to
> > implement.
>
> Trouble is, only superusers can set DEBUG1.
Setting client_min_messages in one's session works, as does "ALTER ROLE myself
SET client_min_messages = debug1". Still, indeed, DEBUG1 is not the usual place
to send a user for information.
> You're right, its more complex than I made out, though that strengthens
> the feeling that we need a way to check what it does before it does it,
> or a way to limit your expectations. Ideally that would be a
> programmatic way, so that pgAdmin et al can issue a warning.
>
> Given your thoughts above, my preference would be for
> EXPLAIN ALTER TABLE to describe the actions that will take place.
That does seem like the best UI. Offhand, I would guess that's a project larger
than the patch series I have here. We'd need to restructure ALTER TABLE into
clear planning and execution stages, if not use the actual planner and executor.
