Hey MA, I've checked, and while not explicitly disallowed, the fact that this could work is more of an implementation detail that shouldn't really be relied on.
The sections and where the instances are on them are organized to maintain the service, and are not supposed to be depended on since they could change.
Even if the mappings are public and fairly stable, there could be a point where a change in the implementation/organization is made -like with this new architecture- and those in-section cross-db joins would stop working.
I'm not saying I will blindly construct cross-wiki queries. Rather, I will only do it after fetching from the db-lists to confirm which ones can be queried cross-wiki. In the case of GUC and XTools Global Contribs, this could mean massive performance improvements. Allow me to paint a picture; we have an account attached to 100 wikis, I want a list of all global edits ordered chronologically. For day-to-day steward life, this is essential so I'd like to find the most efficient route possible, even if it's a little hacky :) So going off of what we're doing now, my high-level vision would be:
1) check db-lists (or from cached result)
2) Check CentralAuth to see which wikis the user has edits on. Here we find there are 100 wikis.
2) Cross-referencing the db-lists, I now know that 75 of the wikis I want to query are on s1, and 25 on s2.
3) For each wiki, I have a subquery to grab all edits by that user on that specific wiki within that slice (may along add WHERE clauses for rev_timestmap, etc.)
4) Take each of those subqueries and wrap it like: (SELECT * FROM ( [subquery1] ) UNION ( [subquery2] ) … ) a ORDER BY rev_timestamp DESC LIMIT 50
5) Do the same for each of the other slices
6) Combine the results from each slice and resequence the edits chronologically, stopping at 50 (the first page of edits to show to the user).
That sounds not like the most fun, but I think it would work. With the current 8 slices, it shouldn't slow it down too terribly (some slices will be faster than others).
Are you discouraging this approach? If I *have* to open and use a separate connection to each of those 100 databases, regardless of the slice, the processing may become much slower. Let's move on to IPs, where we have to check *every* wiki. 900+ separation connections. Again, I'm not sure how I'd get this even set up on my local, as presumably I'd need 900+ open SSH tunnels. Maybe a bash script?
I just want to make sure I've got this right before I start cording. In the end hopefully I'll have a working strategy that I can share with others.