Unless you've built a massive real-time notification system with thousands of distinct channels, it is easy to miss the quadratic performance bottleneck that Postgres used to have in its notification queue. A recent commit fixes that with a spectacular throughput improvement.
The commit in question is 282b1cde9d, which landed recently and targets a future release (likely Postgres 19, though as with all master branch commits, there's a standard caveat that it could be rolled back before release).
The Problem
Previously, when a backend sent a notification (via NOTIFY), Postgres had to determine which other backends were listening to that channel. The implementation involved essentially a linear search or walk through a list for each notification, which became costly when the number of unique channels grew large.
If you had a scenario with thousands of unique channels (e.g., a "table change" notification system where each entity has its own channel), the cost of dispatching notifications would scale quadratically. For a transaction sending N notifications to N different channels, the effort was roughly O(N^2).
The Fix
The optimization introduces a shared hash table to track listeners. Instead of iterating through a list to find interested backends, the notifying process can now look up the channel in the hash table to instantly find who (if anyone) is listening. This implementation uses Partitioned Hash Locking, allowing concurrent LISTEN/UNLISTEN commands without global lock contention.
Additionally, the patch includes an optimization to "direct advance" the queue pointer for listeners that are not interested in the current batch of messages. This is coupled with a Wake-Only-Tail strategy that signals only the backend at the tail of the queue, avoiding "thundering herd" wake-ups and significantly reducing CPU context switches.
Finally, the patch helps observability by adding specific Wait Events, making it easier to spot contention in pg_stat_activity.
Benchmarking Methodology
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