Re: Initial Schema Sync for Logical Replication

First, I think the current publisher doesn't know the version number of

client(subscriber) so we need to check the feasibility of same. Also, having

client's version number checks doesn't seem to be a good idea.

Besides, I thought about the problems that will happen if we try to support

replicating New PG to older PG. The following examples assume that we support the

DDL replication in the mentioned PG.

1) Assume we want to replicate from a newer PG to a older PG where partition

   table has not been introduced. I think even if the publisher is aware of

   that, it doesn't have a good way to transform the partition related command,

   maybe one could say we can transform that to inherit table, but I feel that

   introduces too much complexity.

2) Another example is generated column. To replicate the newer PG which has

   this feature to a older PG without this. I am concerned that is there a way

   to transform this without causing inconsistent behavior.

Even if we decide to simply skip sending such unsupported commands or skip

applying them, then it's likely that the following dml replication will cause

data inconsistency.

So, it seems we cannot completely support this use case, there would be some

limitations. Personally, I am not sure if it's worth introducing complexity to

support it partially.