Sometimes if you do that, you'll never run your ALTER. Just needs enough traffic on the table. In those cases I think the best way is to kill other ongoing queries that block the ALTER, if your app can recover from that.
It might be kind of nice if Postgres had a "force this statement through" option which kills any conflicting processes.
We created something like that in Citus when changing a node's hostname (e.g. during a failover). While node updates should be mutually exclusive with writes (otherwise we might lose them), we didn't want to wait for long-running or possibly frozen writers to release their locks. So after some initial waiting we'd start a background worker to kill anything that was blocking the node update.
