Programming

I'd argue it's better, more efficient, to just store this comment as a document because over and over and over it's going to be needed in that format and anyway you ultimately need to write it to disk as a document.

You're assuming the file system overhead is smaller than the overhead of a database stored on the file system, and that's a pretty bold statement.

The issue isn't storage and retrieval, it's search, it always has been. Your comment being a document or not isn't the hard part or the expensive part. Associating your comment with the post, comments, and likes is where the hard parts come in.

Yeah, you could make every comment a document of sorts and store each one as a file and add an index that points to the files, but you'd be introducing a bunch of overhead caused by the file system.

It's kind of a similar idea for why you don't store a movie as a directory with a picture for each frame. You absolutely could, but there are very good reasons not to do so.

Database are just hyper optimized versions of "storing documents." I think you're severely overestimating the cost of moving a few bits around to change how data is represented.