fix: Throw type error for incomparable join keys (#2098)

Author: TrevorBergeron

bigframes/core/array_value.py

@@ -480,6 +480,14 @@ def relational_join(
         type: typing.Literal["inner", "outer", "left", "right", "cross"] = "inner",
         propogate_order: Optional[bool] = None,
     ) -> typing.Tuple[ArrayValue, typing.Tuple[dict[str, str], dict[str, str]]]:
+        for lcol, rcol in conditions:
+            ltype = self.get_column_type(lcol)
+            rtype = other.get_column_type(rcol)
+            if not bigframes.dtypes.can_compare(ltype, rtype):
+                raise TypeError(
+                    f"Cannot join with non-comparable join key types: {ltype}, {rtype}"
+                )
+
         l_mapping = {  # Identity mapping, only rename right side
             lcol.name: lcol.name for lcol in self.node.ids
         }

bigframes/dtypes.py

@@ -358,6 +358,11 @@ def is_comparable(type_: ExpressionType) -> bool:
     return (type_ is not None) and is_orderable(type_)
 
 
+def can_compare(type1: ExpressionType, type2: ExpressionType) -> bool:
+    coerced_type = coerce_to_common(type1, type2)
+    return is_comparable(coerced_type)
+
+
 def get_struct_fields(type_: ExpressionType) -> dict[str, Dtype]:
     assert isinstance(type_, pd.ArrowDtype)
     assert isinstance(type_.pyarrow_dtype, pa.StructType)

bigframes/operations/type.py

@@ -174,15 +174,7 @@ class CoerceCommon(BinaryTypeSignature):
     def output_type(
         self, left_type: ExpressionType, right_type: ExpressionType
     ) -> ExpressionType:
-        try:
-            return bigframes.dtypes.coerce_to_common(left_type, right_type)
-        except TypeError:
-            pass
-        if bigframes.dtypes.can_coerce(left_type, right_type):
-            return right_type
-        if bigframes.dtypes.can_coerce(right_type, left_type):
-            return left_type
-        raise TypeError(f"Cannot coerce {left_type} and {right_type} to a common type.")
+        return bigframes.dtypes.coerce_to_common(left_type, right_type)
 
 
 @dataclasses.dataclass
@@ -192,8 +184,7 @@ class Comparison(BinaryTypeSignature):
     def output_type(
         self, left_type: ExpressionType, right_type: ExpressionType
     ) -> ExpressionType:
-        common_type = CoerceCommon().output_type(left_type, right_type)
-        if not bigframes.dtypes.is_comparable(common_type):
+        if not bigframes.dtypes.can_compare(left_type, right_type):
             raise TypeError(f"Types {left_type} and {right_type} are not comparable")
         return bigframes.dtypes.BOOL_DTYPE
 

tests/system/small/test_dataframe.py

@@ -3129,8 +3129,6 @@ def test_series_binop_add_different_table(
 @all_joins
 def test_join_same_table(scalars_dfs_maybe_ordered, how):
     bf_df, pd_df = scalars_dfs_maybe_ordered
-    if not bf_df._session._strictly_ordered and how == "cross":
-        pytest.skip("Cross join not supported in partial ordering mode.")
 
     bf_df_a = bf_df.set_index("int64_too")[["string_col", "int64_col"]]
     bf_df_a = bf_df_a.sort_index()
@@ -3153,6 +3151,21 @@ def test_join_same_table(scalars_dfs_maybe_ordered, how):
     assert_pandas_df_equal(bf_result, pd_result, ignore_order=True)
 
 
+def test_join_incompatible_key_type_error(scalars_dfs):
+    bf_df, _ = scalars_dfs
+
+    bf_df_a = bf_df.set_index("int64_too")[["string_col", "int64_col"]]
+    bf_df_a = bf_df_a.sort_index()
+
+    bf_df_b = bf_df.set_index("date_col")[["float64_col"]]
+    bf_df_b = bf_df_b[bf_df_b.float64_col > 0]
+    bf_df_b = bf_df_b.sort_values("float64_col")
+
+    with pytest.raises(TypeError):
+        # joining incompatible date, int columns
+        bf_df_a.join(bf_df_b, how="left")
+
+
 @all_joins
 def test_join_different_table(
     scalars_df_index, scalars_df_2_index, scalars_pandas_df_index, how