Be advised that the work_mem setting (and its deprecated alias sort_mem) are on a per-client basis. So if you have a bunch of people running reports with that setting, you might discover your server running out of memory; that's a really high setting. Generally, if it's only a report or two that need a lot more working memory for sorts, you can do this at the beginning of them instead:
set work_mem='512MB';
so how do I change it back to default (without knowing what the previous val was). I suppose having it in a transaction won't do :P