feat(api): Adjust WorkflowDraftVariable and WorkflowNodeExecutionModel (#20746)

- Add `node_execution_id` column to `WorkflowDraftVariable`, allowing efficient implementation of 
  the "Reset to last run value" feature.
- Add additional index for `WorkflowNodeExecutionModel` to improve the performance of last run lookup.

Closes #20745.

api/models/workflow.py

@@ -16,8 +16,8 @@ if TYPE_CHECKING:
     from models.model import AppMode
 
 import sqlalchemy as sa
-from sqlalchemy import UniqueConstraint, func
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy import Index, PrimaryKeyConstraint, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, declared_attr, mapped_column
 
 from constants import DEFAULT_FILE_NUMBER_LIMITS, HIDDEN_VALUE
 from core.helper import encrypter
@@ -590,28 +590,48 @@ class WorkflowNodeExecutionModel(Base):
     """
 
     __tablename__ = "workflow_node_executions"
-    __table_args__ = (
-        db.PrimaryKeyConstraint("id", name="workflow_node_execution_pkey"),
-        db.Index(
-            "workflow_node_execution_workflow_run_idx",
-            "tenant_id",
-            "app_id",
-            "workflow_id",
-            "triggered_from",
-            "workflow_run_id",
-        ),
-        db.Index(
-            "workflow_node_execution_node_run_idx", "tenant_id", "app_id", "workflow_id", "triggered_from", "node_id"
-        ),
-        db.Index(
-            "workflow_node_execution_id_idx",
-            "tenant_id",
-            "app_id",
-            "workflow_id",
-            "triggered_from",
-            "node_execution_id",
-        ),
-    )
+
+    @declared_attr
+    def __table_args__(cls):  # noqa
+        return (
+            PrimaryKeyConstraint("id", name="workflow_node_execution_pkey"),
+            Index(
+                "workflow_node_execution_workflow_run_idx",
+                "tenant_id",
+                "app_id",
+                "workflow_id",
+                "triggered_from",
+                "workflow_run_id",
+            ),
+            Index(
+                "workflow_node_execution_node_run_idx",
+                "tenant_id",
+                "app_id",
+                "workflow_id",
+                "triggered_from",
+                "node_id",
+            ),
+            Index(
+                "workflow_node_execution_id_idx",
+                "tenant_id",
+                "app_id",
+                "workflow_id",
+                "triggered_from",
+                "node_execution_id",
+            ),
+            Index(
+                # The first argument is the index name,
+                # which we leave as `None`` to allow auto-generation by the ORM.
+                None,
+                cls.tenant_id,
+                cls.workflow_id,
+                cls.node_id,
+                # MyPy may flag the following line because it doesn't recognize that
+                # the `declared_attr` decorator passes the receiving class as the first
+                # argument to this method, allowing us to reference class attributes.
+                cls.created_at.desc(),  # type: ignore
+            ),
+        )
 
     id: Mapped[str] = mapped_column(StringUUID, server_default=db.text("uuid_generate_v4()"))
     tenant_id: Mapped[str] = mapped_column(StringUUID)
@@ -885,14 +905,29 @@ class WorkflowDraftVariable(Base):
 
     selector: Mapped[str] = mapped_column(sa.String(255), nullable=False, name="selector")
 
+    # The data type of this variable's value
     value_type: Mapped[SegmentType] = mapped_column(EnumText(SegmentType, length=20))
-    # JSON string
+
+    # The variable's value serialized as a JSON string
     value: Mapped[str] = mapped_column(sa.Text, nullable=False, name="value")
 
-    # visible
+    # Controls whether the variable should be displayed in the variable inspection panel
     visible: Mapped[bool] = mapped_column(sa.Boolean, nullable=False, default=True)
+
+    # Determines whether this variable can be modified by users
     editable: Mapped[bool] = mapped_column(sa.Boolean, nullable=False, default=False)
 
+    # The `node_execution_id` field identifies the workflow node execution that created this variable.
+    # It corresponds to the `id` field in the `WorkflowNodeExecutionModel` model.
+    #
+    # This field is not `None` for system variables and node variables, and is  `None`
+    # for conversation variables.
+    node_execution_id: Mapped[str | None] = mapped_column(
+        StringUUID,
+        nullable=True,
+        default=None,
+    )
+
     def get_selector(self) -> list[str]:
         selector = json.loads(self.selector)
         if not isinstance(selector, list):