On 12/12/18 3:19 PM, Mike Lissner wrote: > This sounds *very* plausible. So I think there are a few takeaways: > > 1. Should the docs mention that additive changes with NOT NULL > constraints are bad?
It's not the NOT NULL it's the lack of a DEFAULT. In general a column with a NOT NULL and no DEFAULT is going to to bite you sooner or later:) At this point I have gathered enough of those bite marks to just make it my policy to always provide a DEFAULT for a NOT NULL column.
> > 2. Is there a way this could work without completely breaking > replication? For example, should Postgresql realize replication can't > work in this instance and then stop it until schemas are back in sync, > like it does with other incompatible schema changes? That'd be better > than failing in this way and is what I'd expect to happen.
Not sure as there is no requirement that a column has a specified DEFAULT. This is unlike PK and FK constraint violations where the relationship is spelled out. Trying to parse all the possible ways a user could get into trouble would require something on the order of an AI and I don't see that happening anytime soon.
> > 3. Are there other edge cases like this that aren't well documented that > we can expect to creep up on us? If so, should we try to spell out > exactly *which* additive changes *are* OK?
Not that I know of. By their nature edge cases are rare and often are dealt with in the moment and not pushed out to everybody. The only solution I know of is pretesting your schema change/replication setup on a dev installation.
> > This feels like a major "gotcha" to me, and I'm trying to avoid those. I > feel like the docs are pretty lacking here and that others will find > themselves in similarly bad positions.
Logical replication in core(not the pglogical extension) appeared for the first time in version 10. On the crawl/walk/run spectrum it is moving from crawl to walk. The docs will take some time to be more complete. Just for the record my previous post was sketching out a possible scenario not an ironclad answer. If you think the answer is plausible and a 'gotcha' I would file a bug:
