oid caching leads to broken queries after schema changes about diesel HOT 2 OPEN

Thanks for filling this bug report. It's definitively helpful to have a good minimal example for this.

That written: I unfortunately see no good way to change the current behavior without running into massive performance problems. I think this essentially boils down to: Change the schema is only supported as long as the various internal caches in the connection are not populated yet. After that it's not safe to reuse the connection again and expect it to continue to work. You need to establish connections at that point by marking your connection pool as stale and reestablish connections. After all you know at that point you are calling these queries that something might go stale, which is more than diesel knows about that particular queries.

That written: I'm open to accept contributions to improve the current behavior, but I would strongly prefer first to have a suggested design before someone starts to implement something. For me that's currently not a priority (well maybe beside just adding a documentation note about this specific behavior).

Relatedly: why is the cache per-connection? We're using a connection pool. So even if Diesel had a way to flush the cache and re-fill it, it's hard for us to go do that on every connection. It'd be really nice if the cache were distributed via a watch channel or something where we could update a canonical copy and then propagate that out to all the consumers that want it. I guess that might be hard if consumers need to modify it themselves as they run into new types that they need?

Simply because there is no pool in diesel, we just provide support for r2d2 and that does not allow to store these information in the pool itself. That only leaves storing them in the connection. I cannot really comment for other third party pool implementations here. Again that's something where I would accept contributions to change it.

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@fbecart Please not that this issue is not about the prepared statement cache, but about the oid type cache which is something different. So your post here is off topic. Please create a new discussion if you feel that this needs to be discussed.

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