Thanks Adrian for all the help. I filed this as bug #15549. I hope this all helps get logical replication into the "Running" stage.
On Wed, Dec 12, 2018 at 5:06 PM Adrian Klaver <adrian.kla...@aklaver.com> wrote: > 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: > > https://www.postgresql.org/account/login/?next=/account/submitbug/ > > > > > Better schema migration docs would surely help, too. > > > > Mike > > > > > > > -- > Adrian Klaver > adrian.kla...@aklaver.com >
