diff --git a/src/datachain/query/dataset.py b/src/datachain/query/dataset.py
index 8991e73f7..f942b1695 100644
--- a/src/datachain/query/dataset.py
+++ b/src/datachain/query/dataset.py
@@ -1238,30 +1238,27 @@ def _describe(cols: set[str], side: str) -> str:
         return cls(f"Cannot perform union. {'. '.join(parts)}")
 
 
-def _validate_columns(
-    left_columns: Iterable[ColumnElement], right_columns: Iterable[ColumnElement]
-) -> set[str]:
-    left_names = {c.name for c in left_columns}
-    right_names = {c.name for c in right_columns}
-
-    if left_names == right_names:
-        return left_names
-
-    raise UnionSchemaMismatchError.from_column_sets(
-        left_names - right_names,
-        right_names - left_names,
-    )
-
-
 def _order_columns(
     left_columns: Iterable[ColumnElement], right_columns: Iterable[ColumnElement]
 ) -> list[list[ColumnElement]]:
-    column_order = _validate_columns(left_columns, right_columns)
+    left_names = [c.name for c in left_columns]
+    right_names = [c.name for c in right_columns]
+
+    # validate
+    if sorted(left_names) != sorted(right_names):
+        left_names_set = set(left_names)
+        right_names_set = set(right_names)
+        raise UnionSchemaMismatchError.from_column_sets(
+            left_names_set - right_names_set,
+            right_names_set - left_names_set,
+        )
+
+    # Order columns to match left_names order
     column_dicts = [
         {c.name: c for c in columns} for columns in [left_columns, right_columns]
     ]
 
-    return [[d[n] for n in column_order] for d in column_dicts]
+    return [[d[n] for n in left_names] for d in column_dicts]
 
 
 def _drop_system_columns(columns: Iterable[ColumnElement]) -> list[ColumnElement]:
diff --git a/tests/unit/lib/test_datachain.py b/tests/unit/lib/test_datachain.py
index 2998a2898..1527a57ec 100644
--- a/tests/unit/lib/test_datachain.py
+++ b/tests/unit/lib/test_datachain.py
@@ -4555,3 +4555,53 @@ class Signal2(DataModel):
     assert chain.max("signals.signal.i3") == 15
     assert chain.max("signals.signal.f3") == 7.5
     assert chain.max("signals.signal.s3") == "eee"
+
+
+def test_union_does_not_break_schema_order(test_session):
+    class Meta(BaseModel):
+        name: str
+        count: int
+
+    def add_file(key) -> File:
+        return File(path="")
+
+    def add_meta(file) -> Meta:
+        return Meta(name="meta", count=10)
+
+    keys = ["a", "b", "c", "d"]
+    values = [3, 3, 3, 3]
+
+    (
+        dc.read_values(key=keys, val=values, session=test_session)
+        .map(file=add_file)
+        .map(meta=add_meta)
+        .save("ds1")
+    )
+    (
+        dc.read_values(key=keys, val=values, session=test_session)
+        .map(file=add_file)
+        .map(meta=add_meta)
+        .save("ds2")
+    )
+
+    (
+        dc.read_dataset("ds1", session=test_session)
+        .union(dc.read_dataset("ds2", session=test_session))
+        .save("union")
+    )
+
+    dat = test_session.catalog.get_dataset("union")
+    assert list(dat.versions[0].schema.keys()) == [
+        "key",
+        "val",
+        "file__source",
+        "file__path",
+        "file__size",
+        "file__version",
+        "file__etag",
+        "file__is_latest",
+        "file__last_modified",
+        "file__location",
+        "meta__name",
+        "meta__count",
+    ]