When to choose putting logic into PL/pgSQL and when to use an app server

Hi folks,

I started to get into PL/pgSQL a bit as I'm started to work at an
application where I might can make usage of it. But I'm not sure even
those I read a bunch of documentation where is it useful to put logic
into database server and when I shall do the computing inside an
application layer... Does anyone of you have some kind of rule of a
thumb about that?

Cheers,
Frank