Often you want to query some statistic for a user across all sites. For example, you might write a query to find your rejected suggested edits across all Stack Exchange sites.
To implement this, you would join on the network-wide user ID, across the tables corresponding to all the sites. (Doing it "properly" probably requires adding this as a column to each user table.)
Currently, it's possible to do this with a hack, by introspecting the system tables to see what sites exist, and using the output to write a query that joins across all the sites' tables. See:
- Allow cross-site queries in data explorer
- How can I write a cross-site Data Explorer query that joins databases on linked accounts?
But this is hard to use and cumbersome. Furthermore, this makes it difficult to bookmark and share queries because the second query has to be manually updated each time a new site is added to the Data Explorer.
Please add support for doing these joins automatically. Ideally, it would allow arbitrary joins, but even supporting only joining on global ID will make a common use case easier.