I know! I know! It is huge, but from historical reasons, I have this, and I would love to reset it to zero hiding all items in it. Is it possible to do this somehow?

There is no one click way to do this unfortunately.  

Unfortunately, as mentioned above, there isnt a way in which to mass change these within the software at present. 

So do I leave it like this or is there any "manual sql querry" non-software way of doing it?

Manual queries are not OFFICIALLY supported by IPS.  If it matters that much, it may be worth having a third party plugin created to do this. 

Its not something on which we can advise, unfortunately.

Can you recommend me such a plugin created that can do this task? I just need zeroing.

I don’t think one exists. It would need to be created. 

At least the name of a table I should take a peak at?

 

At least the name of a table I should take a peak at?

forums_topics and forums_posts.

Take a look at this topic: 

There will be only difference: you’ll have to replace 2 for -1 but I’m not totally sure; would have to make a quick test. I’d recommend to wait for a confirmation. I should be able to do it in a couple of hours or so.

Thank you so much @Adriano Faria you're a hero!