If I write a function for PostgreSql using PLV8, can I call an url with a get/post request from my PLV8 function?
No, as explained by Milen; use an untrusted PL like PL/perlu, PL/pythonu, PL/javau, etc.
Doing this has the same problem as sending email from a trigger, in that unexpected issues like DNS configuration problems could leave all your database connections busy waiting on HTTP connection attempts so nothing else can get any work done.
Instead, use LISTEN and NOTIFY to wake an external helper script that uses a queue table to manage the requests, as explained in the answer linked above.
No, according to this page and my understanding of "trusted":
PL/v8 is a trusted procedural language that is safe to use, fast to run and easy to develop, powered by V8 JavaScript Engine.
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