We plan to run a maintenance script to delete a bunch of incorrectly imported topics on ptwikibooks soon (https://phabricator.wikimedia.org/T119509#2304693)
It had been tested on beta & locally, and I'm doing a dryrun first & look at the results.
But you never know...
I expect for roughly 25k flow_workflow entries, 75k flow_revision entries, 50k flow_tree_node & flow_tree_revision entries, 190k flow_wiki_ref & 40k flow_ext_ref entries to be deleted (all of them in flowdb, on extension1). There should be no alterations in ptwikibooks db.
They'll be batched & waitForSlaves per 10 topics.
What would be the best backup plan in case something goes wrong (for both databases, just to be sure)? mysqldump?