If your cache fits in Redis then it fits in RAM, if your cache fits in RAM then Postgres will serve it from RAM just as well.

Writes will go to RAM as well if you have synchronous=off.

Not necessarily true. If you're sharing the database with your transaction workload your cache will be paged out eventually.

	▲	jgalt212 3 hours ago \| parent [-]
		This was my take as well, but I'm a MySQL / Redis shop. I really have no idea what tables MySQL has in RAM at any given moment, but with Redis I know what's in RAM.