I work at Hasura.

Hasura uses a novel way of batching similar parameterized subscriptions together and then polls under the hood. This means that if there are 1000 subscribers of similar type of query, then underneath Hasura will only make a single query to Postgres (or few queries depending on the batch size). This approach, which we call "multiplexing" in short, scales really really well. And is also the simplest way to get live updates for _any_ query, no matter how many joins, etc.

We talk more about this approach (and comparisons with other approaches), and benchmarks in this post: https://hasura.io/blog/1-million-active-graphql-subscription...