Any rule of thumb as general as the one I proposed is bound to have exceptions. For one, when the tenants know each other and have formed a consortium to allow

Any rule of thumb as general as the one I proposed is bound to have exceptions. For one, when the tenants know each other and have formed a consortium to allow their data to be aggregated, it's not the kind of "multi-tenant" I had in mind. Now, instead of a bunch of independent renters, we have a condo association. My first instinct in a situation like this would be to ask: "Why not build a separate data warehouse and have some worker processes copy the clients' usage data over there, in batches or in a stream, raw or pre-processed as you prefer? Then the data can be be indexed however you want and JOINed with wild abandon, without further perturbing the individual client systems and their existing, presumably working architecture." Isn't that the old-school IT way? Yeah, I know, terabytes, but a terabyte isn't as scary as it used to be. Of course, the devil is in the details and I'm sure there are reasons.