In many cases, we have to do join other than primary key for the tables as same can't be configured with sync Record type and ultimately, we need to depend on Database views and we loose many capabilities of Sync Record type
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I think that you can already use custom record fields for relationships.
docs.appian.com/.../record-type-relationships.html
Record relationships are there as a compliment to the joins between multiple records. And using relationships much of the features of views are replicable.