To Brion: thanks for your explanation [not copied here]
To all: I overlooked this nice and recent http://bugzilla.wikimedia.org/show_bug.cgi?id=3612 Function to compress or delete old content in database
which appears to be a solution or at least the best starting point to find a solution to my question
to the database experts:
- How can the database of MediaWiki 1.5.x safely be pruned
( in order to actually remove the oldest revisions of Article, Talk, User and User_talk pages ) ?