Thank you very much for taking the time to explain this in such clear detail.

> This virtually means I have one query sent to the FB server for each hierarchy level, constrained to paragraphs having the current paragraph as parent (WHERE IDParent = :IDThisParagraph...).

Can I ask, do you have a separate 'sort' field so the paragraphs come out in the right order or are the ID's ordered to support this?

> The TreeItems/Nodes have a custom property "IDParagraph", and so in the adjacent "Details" screen I can load all details for one paragraph, move it up and down the hierarchy, delete, modify it, add new paragraphs etc.

Great, this nicely keeps a light structure separate from the heavy payload. I would like to do this too.

> I don't exactly know, by the way, what a CTE is.

Its a Common Table Expression and they support recursion inside a query(so server side). I am only just learning about them, but they have been in Firebird since 2.1 (I think)and were covered in the Version 2.1.3 supplement to Helen's old book. I haven't got the new book yet (as I am hoping for a Kindle version) but I am sure they will be in there.

The problem with CTE (according to this presentation http://www.firebirdsql.org/file/community/ppts/fbcon11/FBTrees2011.pdf is that although the hierarchy is maintained, siblings in the result set can be in any order.

I really appreciate you sharing your experiences of this!