odeke-em
diff --git a/‎google/cloud/spanner_v1/_helpers.py
+131-47 b/‎google/cloud/spanner_v1/_helpers.py
+131-47
diff --git a/‎google/cloud/spanner_v1/snapshot.py
+48-6 b/‎google/cloud/spanner_v1/snapshot.py
+48-6
@@ -266,66 +266,69 @@ def _parse_value_pb(value_pb, field_type, field_name, column_info=None):
     :returns: value extracted from value_pb
     :raises ValueError: if unknown type is passed
     """
+    decoder = _get_type_decoder(field_type, field_name, column_info)
+    return _parse_nullable(value_pb, decoder)
+
+
+def _get_type_decoder(field_type, field_name, column_info=None):
+    """Returns a function that converts a Value protobuf to cell data.
+
+    :type field_type: :class:`~google.cloud.spanner_v1.types.Type`
+    :param field_type: type code for the value
+
+    :type field_name: str
+    :param field_name: column name
+
+    :type column_info: dict
+    :param column_info: (Optional) dict of column name and column information.
+            An object where column names as keys and custom objects as corresponding
+            values for deserialization. It's specifically useful for data types like
+            protobuf where deserialization logic is on user-specific code. When provided,
+            the custom object enables deserialization of backend-received column data.
+            If not provided, data remains serialized as bytes for Proto Messages and
+            integer for Proto Enums.
+
+    :rtype: a function that takes a single protobuf value as an input argument
+    :returns: a function that can be used to extract a value from a protobuf value
+    :raises ValueError: if unknown type is passed
+    """
+
     type_code = field_type.code
-    if value_pb.HasField("null_value"):
-        return None
     if type_code == TypeCode.STRING:
-        return value_pb.string_value
+        return _parse_string
     elif type_code == TypeCode.BYTES:
-        return value_pb.string_value.encode("utf8")
+        return _parse_bytes
     elif type_code == TypeCode.BOOL:
-        return value_pb.bool_value
+        return _parse_bool
     elif type_code == TypeCode.INT64:
-        return int(value_pb.string_value)
+        return _parse_int64
     elif type_code == TypeCode.FLOAT64:
-        if value_pb.HasField("string_value"):
-            return float(value_pb.string_value)
-        else:
-            return value_pb.number_value
+        return _parse_float
     elif type_code == TypeCode.FLOAT32:
-        if value_pb.HasField("string_value"):
-            return float(value_pb.string_value)
-        else:
-            return value_pb.number_value
+        return _parse_float
     elif type_code == TypeCode.DATE:
-        return _date_from_iso8601_date(value_pb.string_value)
+        return _parse_date
     elif type_code == TypeCode.TIMESTAMP:
-        DatetimeWithNanoseconds = datetime_helpers.DatetimeWithNanoseconds
-        return DatetimeWithNanoseconds.from_rfc3339(value_pb.string_value)
-    elif type_code == TypeCode.ARRAY:
-        return [
-            _parse_value_pb(
-                item_pb, field_type.array_element_type, field_name, column_info
-            )
-            for item_pb in value_pb.list_value.values
-        ]
-    elif type_code == TypeCode.STRUCT:
-        return [
-            _parse_value_pb(
-                item_pb, field_type.struct_type.fields[i].type_, field_name, column_info
-            )
-            for (i, item_pb) in enumerate(value_pb.list_value.values)
-        ]
+        return _parse_timestamp
     elif type_code == TypeCode.NUMERIC:
-        return decimal.Decimal(value_pb.string_value)
+        return _parse_numeric
     elif type_code == TypeCode.JSON:
-        return JsonObject.from_str(value_pb.string_value)
+        return _parse_json
     elif type_code == TypeCode.PROTO:
-        bytes_value = base64.b64decode(value_pb.string_value)
-        if column_info is not None and column_info.get(field_name) is not None:
-            default_proto_message = column_info.get(field_name)
-            if isinstance(default_proto_message, Message):
-                proto_message = type(default_proto_message)()
-                proto_message.ParseFromString(bytes_value)
-                return proto_message
-        return bytes_value
+        return lambda value_pb: _parse_proto(value_pb, column_info, field_name)
     elif type_code == TypeCode.ENUM:
-        int_value = int(value_pb.string_value)
-        if column_info is not None and column_info.get(field_name) is not None:
-            proto_enum = column_info.get(field_name)
-            if isinstance(proto_enum, EnumTypeWrapper):
-                return proto_enum.Name(int_value)
-        return int_value
+        return lambda value_pb: _parse_proto_enum(value_pb, column_info, field_name)
+    elif type_code == TypeCode.ARRAY:
+        element_decoder = _get_type_decoder(
+            field_type.array_element_type, field_name, column_info
+        )
+        return lambda value_pb: _parse_array(value_pb, element_decoder)
+    elif type_code == TypeCode.STRUCT:
+        element_decoders = [
+            _get_type_decoder(item_field.type_, field_name, column_info)
+            for item_field in field_type.struct_type.fields
+        ]
+        return lambda value_pb: _parse_struct(value_pb, element_decoders)
     else:
         raise ValueError("Unknown type: %s" % (field_type,))
 
@@ -351,6 +354,87 @@ def _parse_list_value_pbs(rows, row_type):
     return result
 
 
+def _parse_string(value_pb) -> str:
+    return value_pb.string_value
+
+
+def _parse_bytes(value_pb):
+    return value_pb.string_value.encode("utf8")
+
+
+def _parse_bool(value_pb) -> bool:
+    return value_pb.bool_value
+
+
+def _parse_int64(value_pb) -> int:
+    return int(value_pb.string_value)
+
+
+def _parse_float(value_pb) -> float:
+    if value_pb.HasField("string_value"):
+        return float(value_pb.string_value)
+    else:
+        return value_pb.number_value
+
+
+def _parse_date(value_pb):
+    return _date_from_iso8601_date(value_pb.string_value)
+
+
+def _parse_timestamp(value_pb):
+    DatetimeWithNanoseconds = datetime_helpers.DatetimeWithNanoseconds
+    return DatetimeWithNanoseconds.from_rfc3339(value_pb.string_value)
+
+
+def _parse_numeric(value_pb):
+    return decimal.Decimal(value_pb.string_value)
+
+
+def _parse_json(value_pb):
+    return JsonObject.from_str(value_pb.string_value)
+
+
+def _parse_proto(value_pb, column_info, field_name):
+    bytes_value = base64.b64decode(value_pb.string_value)
+    if column_info is not None and column_info.get(field_name) is not None:
+        default_proto_message = column_info.get(field_name)
+        if isinstance(default_proto_message, Message):
+            proto_message = type(default_proto_message)()
+            proto_message.ParseFromString(bytes_value)
+            return proto_message
+    return bytes_value
+
+
+def _parse_proto_enum(value_pb, column_info, field_name):
+    int_value = int(value_pb.string_value)
+    if column_info is not None and column_info.get(field_name) is not None:
+        proto_enum = column_info.get(field_name)
+        if isinstance(proto_enum, EnumTypeWrapper):
+            return proto_enum.Name(int_value)
+    return int_value
+
+
+def _parse_array(value_pb, element_decoder) -> []:
+    return [
+        _parse_nullable(item_pb, element_decoder)
+        for item_pb in value_pb.list_value.values
+    ]
+
+
+def _parse_struct(value_pb, element_decoders):
+    return [
+        _parse_nullable(item_pb, element_decoders[i])
+        for (i, item_pb) in enumerate(value_pb.list_value.values)
+    ]
+
+
+def _parse_nullable(value_pb, decoder):
+    if value_pb.HasField("null_value"):
+        return None
+    else:
+        return decoder(value_pb)
+
+
 class _SessionWrapper(object):
     """Base class for objects wrapping a session.
 
 
@@ -192,6 +192,7 @@ def read(
         retry=gapic_v1.method.DEFAULT,
         timeout=gapic_v1.method.DEFAULT,
         column_info=None,
+        lazy_decode=False,
     ):
         """Perform a ``StreamingRead`` API request for rows in a table.
 
@@ -255,6 +256,18 @@ def read(
             If not provided, data remains serialized as bytes for Proto Messages and
             integer for Proto Enums.
 
+        :type lazy_decode: bool
+        :param lazy_decode:
+            (Optional) If this argument is set to ``true``, the iterator
+            returns the underlying protobuf values instead of decoded Python
+            objects. This reduces the time that is needed to iterate through
+            large result sets. The application is responsible for decoding
+            the data that is needed. The returned row iterator contains two
+            functions that can be used for this. ``iterator.decode_row(row)``
+            decodes all the columns in the given row to an array of Python
+            objects. ``iterator.decode_column(row, column_index)`` decodes one
+            specific column in the given row.
+
         :rtype: :class:`~google.cloud.spanner_v1.streamed.StreamedResultSet`
         :returns: a result set instance which can be used to consume rows.
 
@@ -330,10 +343,15 @@ def read(
                 self._read_request_count += 1
                 if self._multi_use:
                     return StreamedResultSet(
-                        iterator, source=self, column_info=column_info
+                        iterator,
+                        source=self,
+                        column_info=column_info,
+                        lazy_decode=lazy_decode,
                     )
                 else:
-                    return StreamedResultSet(iterator, column_info=column_info)
+                    return StreamedResultSet(
+                        iterator, column_info=column_info, lazy_decode=lazy_decode
+                    )
         else:
             iterator = _restart_on_unavailable(
                 restart,
@@ -348,9 +366,13 @@ def read(
         self._read_request_count += 1
 
         if self._multi_use:
-            return StreamedResultSet(iterator, source=self, column_info=column_info)
+            return StreamedResultSet(
+                iterator, source=self, column_info=column_info, lazy_decode=lazy_decode
+            )
         else:
-            return StreamedResultSet(iterator, column_info=column_info)
+            return StreamedResultSet(
+                iterator, column_info=column_info, lazy_decode=lazy_decode
+            )
 
     def execute_sql(
         self,
@@ -366,6 +388,7 @@ def execute_sql(
         data_boost_enabled=False,
         directed_read_options=None,
         column_info=None,
+        lazy_decode=False,
     ):
         """Perform an ``ExecuteStreamingSql`` API request.
 
@@ -438,6 +461,18 @@ def execute_sql(
             If not provided, data remains serialized as bytes for Proto Messages and
             integer for Proto Enums.
 
+        :type lazy_decode: bool
+        :param lazy_decode:
+            (Optional) If this argument is set to ``true``, the iterator
+            returns the underlying protobuf values instead of decoded Python
+            objects. This reduces the time that is needed to iterate through
+            large result sets. The application is responsible for decoding
+            the data that is needed. The returned row iterator contains two
+            functions that can be used for this. ``iterator.decode_row(row)``
+            decodes all the columns in the given row to an array of Python
+            objects. ``iterator.decode_column(row, column_index)`` decodes one
+            specific column in the given row.
+
         :raises ValueError:
             for reuse of single-use snapshots, or if a transaction ID is
             already pending for multiple-use snapshots.
@@ -517,6 +552,7 @@ def execute_sql(
                     trace_attributes,
                     column_info,
                     observability_options,
+                    lazy_decode=lazy_decode,
                 )
         else:
             return self._get_streamed_result_set(
@@ -525,6 +561,7 @@ def execute_sql(
                 trace_attributes,
                 column_info,
                 observability_options,
+                lazy_decode=lazy_decode,
             )
 
     def _get_streamed_result_set(
@@ -534,6 +571,7 @@ def _get_streamed_result_set(
         trace_attributes,
         column_info,
         observability_options=None,
+        lazy_decode=False,
     ):
         iterator = _restart_on_unavailable(
             restart,
@@ -548,9 +586,13 @@ def _get_streamed_result_set(
         self._execute_sql_count += 1
 
         if self._multi_use:
-            return StreamedResultSet(iterator, source=self, column_info=column_info)
+            return StreamedResultSet(
+                iterator, source=self, column_info=column_info, lazy_decode=lazy_decode
+            )
         else:
-            return StreamedResultSet(iterator, column_info=column_info)
+            return StreamedResultSet(
+                iterator, column_info=column_info, lazy_decode=lazy_decode
+            )
 
     def partition_read(
         self,