As far as I know there is no way to tell Postgres to keep a particular index or table in memory, which is one reason to be weary of using one PG instance for ma

As far as I know there is no way to tell Postgres to keep a particular index or table in memory, which is one reason to be weary of using one PG instance for many varied workloads. You might solve this by earmarking workload-specific replicas, though. If you can keep your entire working set in memory, though, then it probably doesn't matter that much.