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 ) ?
>
>