Cross-Database Joins: The Coupling You Can't See Until You Split

TL;DR A join across two schemas on the same MySQL server costs nothing to write, which is exactly why nobody counts it. Every one of those joins is a hard coupling that pins both schemas to one instance, so the day you need to split for scale, the joins are the wall. And the referential integrity you assumed the boundary carried was never enforced to begin with. Somewhere in the codebase, since…