refactor(ir): impure values are never dereferencible (#9023)

All impure values passed to the API methods are treated as separate
impure invocations:

```py
impure = ibis.random()
t1 = t.mutate(y=impure)
t2 = t1.mutate(z=impure.cast("string"))
```

now produce

```
r0 := UnboundTable: unbound_table_0
  a int64
  b string

r1 := Project[r0]
  a: r0.a
  b: r0.b
  y: RandomScalar()

Project[r1]
  a: r1.a
  b: r1.b
  y: r1.y
  z: Cast(RandomScalar(), to=string)
```

ibis/backends/pandas/executor.py

@@ -110,6 +110,10 @@ def visit(cls, op: ops.Value, **operands):
         if func := cls.kernels.generic.get(typ):
             return cls.generic(func, **kwargs)
 
+        if len(operands) < 1:
+            raise OperationNotDefinedError(
+                f"No implementation found for operation {typ}"
+            )
         _, *rest = operands.values()
         is_multi_arg = bool(rest)
         is_multi_column = any_of(rest, pd.Series)

ibis/expr/operations/generic.py

@@ -185,13 +185,7 @@ class Constant(Scalar, Singleton):
 
 @public
 class Impure(Value):
-    _counter = itertools.count()
-    uid: Optional[int] = None
-
-    def __init__(self, uid, **kwargs):
-        if uid is None:
-            uid = next(self._counter)
-        super().__init__(uid=uid, **kwargs)
+    pass
 
 
 @public

ibis/expr/operations/tests/test_generic.py

@@ -146,10 +146,6 @@ def test_NULL():
 
 
 @pytest.mark.parametrize("op", [ops.RandomScalar, ops.RandomUUID])
-def test_unique_impure_values(op):
-    assert op() != op()
-    assert hash(op()) != hash(op())
-
-    node = op()
-    other = node.copy()
-    assert node == other
+def test_impure_values_are_equal(op):
+    assert op() == op()
+    assert hash(op()) == hash(op())

ibis/expr/tests/test_newrels.py

@@ -1618,7 +1618,7 @@ def test_impure_operation_dereferencing(func):
 
     expected = ops.Project(
         parent=t1,
-        values={"x": t1.x, "y": t1.y, "z": t1.y.cast("string")},
+        values={"x": t1.x, "y": t1.y, "z": impure.cast("string")},
     )
     assert t2.op() == expected
 

ibis/expr/types/relations.py

@@ -169,7 +169,7 @@ def dereference_mapping(parents):
                 while isinstance(v, ops.Field) and v not in mapping:
                     mapping[v] = ops.Field(parent, k)
                     v = v.rel.values.get(v.name)
-            elif v not in mapping:
+            elif v not in mapping and not v.find(ops.Impure):
                 # do not dereference literal expressions
                 mapping[v] = ops.Field(parent, k)