On Wed, Jun 11, 2014 at 3:26 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> If we didn't have mechanisms like this, we'd have far worse hazards from
> ALTER TABLE than whether the planner made an incorrect join optimization.
> Consider ALTER COLUMN TYPE for instance.

Obviously not general cases of ALTER COLUMN TYPE but dropping a NULL
constraint seems like the kind of change targeted by Simon's "reduce
lock strength" patch that I'm sure he's still interested in. I think
that patch, while full of dragons to steer around, is something that
will keep coming up again and again in the future. It's a huge
operational risk that even these short exclusive locks can cause a
huge production outage if they happen to get queued up behind a
reporting query.

I don't think it changes anything for this patch -- right now the
world is arranged the way Tom described -- but it's something to keep
in mind when we talk about lock strength reduction and the impact on
existing queries. For example if there's an UPDATE query in repeatable
read mode that has an IN clause like this and was optimized
accordingly then any lock strength reduction patch would have to
beware that an ALTER TABLE that dropped the NULL clause might impact
the update query.

Incidentally, Oracle has a feature for online schema changes that we
might end up having to implement something similar. The good news is
we have the infrastructure to maybe do it. The idea is to start
capturing all the changes to the table using something like our
logical changeset extraction. Then do the equivalent of "create
newtable as select ... from oldtable" to create the new schema, then
start replaying the accumulated changes to the new table. Eventually
when the change queue drains then get an exclusive lock, drain any new
changes, and swap in the new table with the new schema.


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