Currently, commonswiki_p.page is accessible and JOINable as commonswiki_f_p.page from within - for example - enwiki_p and dewiki_p. This should be extended to:
- all target databases (for example, zhwiki_p has no link commonswiki_f_p),
- all source databases (not just Commons), and
- all source tables in each of the source databases.
Once this is set up and automated, its usage and limitations (i. e. JOIN only on indexes of the source table, etc.) need to be documented.