The reason you build cache for most purposes, is fast data retrieval. A major component of "fast" is the latency of a disk read vs a memory read. Redis and memc

The reason you build cache for most purposes, is fast data retrieval. A major component of "fast" is the latency of a disk read vs a memory read. Redis and memcached are in memory by design, so entirety of your cached table will be in memory. When you design a cache using postgres (assuming on an RDS instance), although data will be cached in memory based on access patterns, I don't believe there is a direct way to control what data gets cached, which makes this implementation slower for a lot of real world use cases. This gets even slower when you have a distributed DB like AWS Aurora Postgres because your data on disk can be on different EBS volumes, so bringing it in memory can be slower than even RDS Postgres.