Indeed. Even when you have the full identity graph in warehouse and just want to assign a cannonical-ID (by doing a transitive closure), it is not easy in SQL. We wrote a blog on it (sorry for the shameless plug) https://rudderstack.com/blog/identity-graph-and-identity-res...

Creating the ID graph is a next level problem altogether!!. How do you know a record in Salesforce is the same as the anonymous visitor on your website. Requires joining across at-least 3 (possibly more IDs) - anonymousID, userID email (if the user signs up) and Salesforce record email.

Should the data pipe do this automatically? If not, what API abstraction should be exposed to the user?