datalog: bipartite friends-with-hobby join test (229/229)

lib/datalog/tests/eval.sx

@@ -273,6 +273,18 @@
           (dl-saturated? db))
         false)
 
+      ;; Bipartite-style join: pair-of-friends who share a hobby.
+      ;; Three-relation join exercising the planner's join order.
+      (dl-et-test-set! "bipartite friends-with-hobby"
+        (dl-query
+          (dl-program
+            "hobby(alice, climb). hobby(bob, paint).
+             hobby(carol, climb).
+             friend(alice, carol). friend(bob, alice).
+             match(A, B, H) :- friend(A, B), hobby(A, H), hobby(B, H).")
+          (list (quote match) (quote A) (quote B) (quote H)))
+        (list {:A (quote alice) :B (quote carol) :H (quote climb)}))
+
       ;; Repeated variable (diagonal): p(X, X) only matches tuples
       ;; whose two args are equal. The unifier handles this via the
       ;; subst chain — first occurrence binds X, second occurrence