[simonw]

Prove that you need the extra speed.

Run benchmarks that show that, for your application under your expected best-case loads, using Redis for caching instead of PostgreSQL provides a meaningful improvement.

If it doesn't provide a meaningful improvement, stick with PostgreSQL.

[calderarrow]

This is the proper approach when deciding whether to use any type of tool or technology. Is the increased amount of cognitive overhead for someone with minimal exposure to your system (who will have to maintain it when you’ve moved on) worth the increased performance on a dollars-per-hour basis? If so, it may be a good option. If not, it doesn’t matter how much better the relative performance is.

[christophilus]

Yep. And often, if you do find a need for a cache, you can get away with an in-app cache before moving to something like Redis.