> I.e., if you are generating reports, running aggregations over a large amount of data you definitely need some parallelism and Postgres isn't designed to handle these loads (certainly not petabytes). Even aggregating 100's of GB probably requires (or at least is more cost effective using) multiple machines.

Aggregating 100s of GB isn't much of a problem for PG these days. Yes, you can be faster - obviously - but it works quite well. And the price for separate systems (duplicated infrastructure, duplicated data, out-of-sync systems, ...) is noticable as well.

But yea, for many petabytes of data you either have to go to an entirely different system, or use something like Citus.

Disclaimer: I work on PG, and I used to work for Citus. So I'm definitely biased.