[jasonwatkinspdx]

The buffer pool in a rdbms ends up intimately connected with the concurrency control and durability protocols. There's also a variety of tradeoffs in how to handle conflicts between transactions (steal vs no steal, force vs no force, etc). You need deadlock detection or prevention. That creates a necessary minimum of complexity and overhead.

By comparison an in memory kv cache is much more streamlined. They basically just need to move bytes from a hash table to a network socket as fast as possible, with no transactional concerns.

The semantics matter as well. PostgreSQL has to assume all data needs to be retained. Memcached can always just throw something away. Redis persistence is best effort with an explicit loss window. That has enormous practical implications on their internals.

So in practical terms this means they're in different universes performance wise. If your workload is compatible with a kv cache semantically, adding memcached to your infrastructure will probably result in a savings overall.