If a paper is accepted in poster session of a conference or workshop such that only one page of the article will be published in proceedings, how this paper can be credible? (in particular in computer science and in a workshop like this: http://deic.uab.cat/conferences/cbt/cbt2018/)
I'm not sure what you mean by credible. If you are a student, especially, being in a poster session is a really good opportunity to meet people in the field and impress them with your brilliance and future prospects.
As one of the senior professors who used to wander around poster sessions, I was honored to meet such new and upcoming researchers and learn from them. If I had anything useful to say, perhaps they gained something as well. You can usually, at a poster session, distribute a longer version of your work with contact information.
The same goes, I think, for young faculty, though to a lesser extent. But still, use it as a chance to make contacts and extend your reach.
The "paper" in a poster session is probably an abstract, rather than an actual paper. It is a record of the poster presentation, but may or may not have been peer-reviewed. Obviously you would not want to use it as a primary source of results or data but it could be used as a demonstration that a certain topic is being researched.