I believe the main, and maybe only, concern is the bloating of the system catalog tables since you are constantly adding and removing records. Yes, they will be vacuumed but vacuuming and bloat on catalog tables slows every single query down to some, degree since every query has to lookup its objects is those catalogs. Though caching probably alleviates some of that
Yes, that's exactly the concern I heard, thanks for reminding me.
If I want to e.g. temporarily store a "setof records" or a "table" result in a variable as part of a calculation in a plpgsql function, do I have any other option than CREATE TEMPORARY TABLE? It didn't seem to work when I DECLAREd a variable of type "setof table_name" or "setof table_name%rowtype", and then SELECT INTO it.
