[montanalow]

RAM and the Postgres shared buffers, as well OS page cache can also be important factors when there is an active working subset, e.g. the active sessions on a website might be reused hundreds of times per session, with only relatively small portion of them active at any time.

Shared RAM across all cores can be a reason to use fewer larger machines rather than more smaller ones in such a case. Postgres does give you options either way though.

[derefr]

Yup. In both cases, I was presuming a worst-case workload — e.g. an OLAP workload where you have to read all the data in the DB to answer a query, and "all the data" is far larger than RAM; or 10k concurrent queries from different tenants who all care about different things. That's really where bottlenecks will hit you. "More RAM" does help when your problems aren't worst-case problems. :)