I've been looking into PgDog for sharding a 40TB Postgres database atm vs building something ourselves. This could be a good opportunity to collaborate because what we need is something more like Vitess for PostgreSQL. The scatter gather stuff is great but what we really need is config management via something like etcd, shard splitting, best-effort transactions for doing schema changes across all shards etc.
Almost totally unrelated but have you had good success using pg_query.rs to re-write queries? Maybe I misunderstood how pg_query.rs works but re-writing an AST seems like a nightmare with how the AST types don't really support mutability or deep cloning. I ended up using the sqlparser crate which supports mutability via Visitors. I have a side project I'm chipping away at to build online schema change for PG using shadow tables and logical replication ala gh-ost.
Jake