Generate schema like uml diagrams from existing db

Hello friends,
Thanks for reading this post.
I have a very huge database with many relationships among tables. I want to generate a schema diagram (something like a uml diagram) from the database. Is there any tool that does that or any postgresql command that can export a file in some format?

I have been searching the internet but only get commercial versions of softwares, but I am looking for something open source to so that I can reuse it.

Please respond if you have an answer to the above question.

Thanking you.

With warm regards.

Vivek J. Joshi.

vivek@staff.ownmail.com