datachain: support mutating existing column

src/datachain/lib/dc.py

@@ -1111,14 +1111,6 @@ def mutate(self, **kwargs) -> "Self":
         )
         ```
         """
-        existing_columns = set(self.signals_schema.values.keys())
-        for col_name in kwargs:
-            if col_name in existing_columns:
-                raise DataChainColumnError(
-                    col_name,
-                    "Cannot modify existing column with mutate(). "
-                    "Use a different name for the new column.",
-                )
         for col_name, expr in kwargs.items():
             if not isinstance(expr, (Column, Func)) and isinstance(expr.type, NullType):
                 raise DataChainColumnError(

src/datachain/lib/signal_schema.py

@@ -20,6 +20,7 @@
 )
 
 from pydantic import BaseModel, create_model
+from sqlalchemy import ColumnElement
 from typing_extensions import Literal as LiteralEx
 
 from datachain.lib.convert.python_to_sql import python_to_sql
@@ -491,16 +492,14 @@ def mutate(self, args_map: dict) -> "SignalSchema":
                 # renaming existing signal
                 del new_values[value.name]
                 new_values[name] = self.values[value.name]
-            elif name in self.values:
-                # changing the type of existing signal, e.g File -> ImageFile
-                del new_values[name]
-                new_values[name] = args_map[name]
             elif isinstance(value, Func):
                 # adding new signal with function
                 new_values[name] = value.get_result_type(self)
-            else:
+            elif isinstance(value, ColumnElement):
                 # adding new signal
                 new_values[name] = sql_to_python(value)
+            else:
+                new_values[name] = value
 
         return SignalSchema(new_values)
 

src/datachain/query/dataset.py

@@ -10,6 +10,7 @@
 from collections.abc import Generator, Iterable, Iterator, Sequence
 from copy import copy
 from functools import wraps
+from secrets import token_hex
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -720,10 +721,17 @@ class SQLMutate(SQLClause):
 
     def apply_sql_clause(self, query: Select) -> Select:
         original_subquery = query.subquery()
+        to_mutate = {c.name for c in self.args}
+
+        prefix = f"mutate{token_hex(8)}_"
+        cols = [
+            c.label(prefix + c.name) if c.name in to_mutate else c
+            for c in original_subquery.c
+        ]
         # this is needed for new column to be used in clauses
         # like ORDER BY, otherwise new column is not recognized
         subquery = (
-            sqlalchemy.select(*original_subquery.c, *self.args)
+            sqlalchemy.select(*cols, *self.args)
             .select_from(original_subquery)
             .subquery()
         )

tests/func/test_datachain.py

@@ -24,7 +24,7 @@
 from datachain.lib.listing import LISTING_TTL, is_listing_dataset, parse_listing_uri
 from datachain.lib.tar import process_tar
 from datachain.lib.udf import Mapper
-from datachain.lib.utils import DataChainColumnError, DataChainError
+from datachain.lib.utils import DataChainError
 from datachain.query.dataset import QueryStep
 from datachain.sql.functions import path as pathfunc
 from datachain.sql.functions.array import cosine_distance, euclidean_distance
@@ -491,15 +491,9 @@ def test_from_storage_check_rows(tmp_dir, test_session):
 
 def test_mutate_existing_column(test_session):
     ds = DataChain.from_values(ids=[1, 2, 3], session=test_session)
+    ds = ds.mutate(ids=Column("ids") + 1)
 
-    with pytest.raises(DataChainColumnError) as excinfo:
-        ds.mutate(ids=Column("ids") + 1)
-
-    assert (
-        str(excinfo.value)
-        == "Error for column ids: Cannot modify existing column with mutate()."
-        " Use a different name for the new column."
-    )
+    assert list(ds.collect()) == [(2,), (3,), (4,)]
 
 
 @pytest.mark.parametrize(