Hi Hans-Jürgen,
it seems to me that function db:open() and collection() are equivalent. [...]
the main difference of the collection() and doc() functions is that they can also access local resources: you may as well use them to create a temporary main-memory collection from all files found at the given local file path [1]. In other words, db:open() will be a little bit more efficient, and cause no surprises, if you know that your files are stored in a database.
Hope this helps, Christian