select data
from this_table and this_table and this_table and that_table
where there was no previously defined relationship
but users need this data with the new relationship
and have it done in a few minutes
Wait a minute. Unless you already built RDBMS tables with that relationship in mind, you are just as screwed there as you are in Couch. And if you are talking about the time it takes Couch to build a new index for some custom view, I'm sure it's nominal compared to the number of times you feel the need to change or create one.
now in the real world: