fix(sql): support set operations wrapping subqueries

ibis/backends/sql/compiler.py

@@ -1215,10 +1215,10 @@ def visit_Sort(self, op, *, parent, keys):
             return sg.select(STAR).from_(parent).order_by(*keys)
 
     def visit_Union(self, op, *, left, right, distinct):
-        if isinstance(left, sge.Table):
+        if isinstance(left, (sge.Table, sge.Subquery)):
             left = sg.select(STAR).from_(left)
 
-        if isinstance(right, sge.Table):
+        if isinstance(right, (sge.Table, sge.Subquery)):
             right = sg.select(STAR).from_(right)
 
         return sg.union(
@@ -1228,10 +1228,10 @@ def visit_Union(self, op, *, left, right, distinct):
         )
 
     def visit_Intersection(self, op, *, left, right, distinct):
-        if isinstance(left, sge.Table):
+        if isinstance(left, (sge.Table, sge.Subquery)):
             left = sg.select(STAR).from_(left)
 
-        if isinstance(right, sge.Table):
+        if isinstance(right, (sge.Table, sge.Subquery)):
             right = sg.select(STAR).from_(right)
 
         return sg.intersect(
@@ -1241,10 +1241,10 @@ def visit_Intersection(self, op, *, left, right, distinct):
         )
 
     def visit_Difference(self, op, *, left, right, distinct):
-        if isinstance(left, sge.Table):
+        if isinstance(left, (sge.Table, sge.Subquery)):
             left = sg.select(STAR).from_(left)
 
-        if isinstance(right, sge.Table):
+        if isinstance(right, (sge.Table, sge.Subquery)):
             right = sg.select(STAR).from_(right)
 
         return sg.except_(

ibis/backends/tests/test_set_ops.py

@@ -192,9 +192,13 @@ def test_table_set_operations_api(alltypes, method):
         False,
     ],
 )
-def test_top_level_union(backend, con, alltypes, distinct):
+@pytest.mark.parametrize("ordered", [False, True])
+def test_top_level_union(backend, con, alltypes, distinct, ordered):
     t1 = alltypes.select(a="bigint_col").filter(lambda t: t.a == 10).distinct()
     t2 = alltypes.select(a="bigint_col").filter(lambda t: t.a == 20).distinct()
+    if ordered:
+        t1 = t1.order_by("a")
+        t2 = t2.order_by("a")
     expr = t1.union(t2, distinct=distinct).limit(2)
     result = con.execute(expr)
     expected = pd.DataFrame({"a": [10, 20]})
@@ -237,10 +241,11 @@ def test_top_level_union(backend, con, alltypes, distinct):
     ],
     ids=["intersect", "difference"],
 )
+@pytest.mark.parametrize("ordered", [False, True])
 @pytest.mark.notimpl(["polars"], raises=com.OperationNotDefinedError)
 @pytest.mark.broken(["druid"], raises=PyDruidProgrammingError)
 def test_top_level_intersect_difference(
-    backend, con, alltypes, distinct, opname, expected
+    backend, con, alltypes, distinct, opname, expected, ordered
 ):
     t1 = (
         alltypes.select(a="bigint_col")
@@ -252,6 +257,9 @@ def test_top_level_intersect_difference(
         .filter(lambda t: (t.a == 20) | (t.a == 30))
         .distinct()
     )
+    if ordered:
+        t1 = t1.order_by("a")
+        t2 = t2.order_by("a")
     op = getattr(t1, opname)
     expr = op(t2, distinct=distinct).limit(2)
     result = con.execute(expr)