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This patch reworks LISTEN/NOTIFY to avoid waking backends that have no need to process the notification messages we just sent. The primary change is to create a shared hash table that tracks which processes are listening to which channels (where a "channel" is defined by a database OID and channel name). This allows a notifying process to accurately determine which listeners are interested, replacing the previous weak approximation that listeners in other databases couldn't be interested. Secondly, if a listener is known not to be interested and is currently stopped at the old queue head, we avoid waking it at all and just directly advance its queue pointer past the notifications we inserted. These changes permit very significant improvements (integer multiples) in NOTIFY throughput, as well as a noticeable reduction in latency, when there are many listeners but only a few are interested in any specific message. There is no improvement for the simplest case where every listener reads every message, but any loss seems below the noise level. Author: Joel Jacobson <joel@compiler.org> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/6899c044-4a82-49be-8117-e6f669765f7e@app.fastmail.com