WalShadow: Sub-second Postgres replication to ClickHouse from physical WAL

They're not using `wal_level = logical`, which has been the "friendly" way of doing CDC on Postgres since ever, but are going straight to `wal_level = replica` which, afaik, has never really been used to build something atop of except Postgres' own replication.

This is very interesting. I'd never have guessed that it'd make such a difference. I also bet this is the sort of thing that would have never end up being implemented without access to coding agents. Having to figure out these protocol-level details is no longer the huge time sink it was

Does this work with toast stored unchanged differently than logical? Or is it the same in terms need to merge with some last seen state of toasted field to project the whole of a changed tupled? Always a pita.

would this work with a postgres hosted on supabase? for our company brain mcp thing we mirror postgres data (along with server logs, banking data, slack, etc) in clickhouse but took a very inefficient polling approach since it works with any read-only db connection string. always looking to improve that and get rid of latency though.