diff --git a/luigi/execution_summary.py b/luigi/execution_summary.py
index e818a1ed14..a420816124 100644
--- a/luigi/execution_summary.py
+++ b/luigi/execution_summary.py
@@ -94,24 +94,45 @@ def _partition_tasks(worker):
     Still_pending_not_ext is only used to get upstream_failure, upstream_missing_dependency and run_by_other_worker
     """
     task_history = worker._add_task_history
-    pending_tasks = {task for (task, status, ext) in task_history if status == 'PENDING'}
-    set_tasks = {}
-    set_tasks["completed"] = {task for (task, status, ext) in task_history if status == 'DONE' and task in pending_tasks}
-    set_tasks["already_done"] = {task for (task, status, ext) in task_history
-                                 if status == 'DONE' and task not in pending_tasks and task not in set_tasks["completed"]}
-    set_tasks["ever_failed"] = {task for (task, status, ext) in task_history if status == 'FAILED'}
-    set_tasks["failed"] = set_tasks["ever_failed"] - set_tasks["completed"]
-    set_tasks["scheduling_error"] = {task for (task, status, ext) in task_history if status == 'UNKNOWN'}
-    set_tasks["still_pending_ext"] = {task for (task, status, ext) in task_history
-                                      if status == 'PENDING' and task not in set_tasks["ever_failed"] and task not in set_tasks["completed"] and not ext}
-    set_tasks["still_pending_not_ext"] = {task for (task, status, ext) in task_history
-                                          if status == 'PENDING' and task not in set_tasks["ever_failed"] and task not in set_tasks["completed"] and ext}
-    set_tasks["run_by_other_worker"] = set()
-    set_tasks["upstream_failure"] = set()
-    set_tasks["upstream_missing_dependency"] = set()
-    set_tasks["upstream_run_by_other_worker"] = set()
-    set_tasks["upstream_scheduling_error"] = set()
-    set_tasks["not_run"] = set()
+
+    set_tasks = {
+        "completed": set(),
+        "already_done": set(),
+        "ever_failed": set(),
+        "failed": set(),
+        "scheduling_error": set(),
+        "still_pending_ext": set(),
+        "still_pending_not_ext": set(),
+        "run_by_other_worker": set(),
+        "upstream_failure": set(),
+        "upstream_missing_dependency": set(),
+        "upstream_run_by_other_worker": set(),
+        "upstream_scheduling_error": set(),
+        "not_run": set()
+    }
+
+    pending_tasks = set()
+
+    for task, status, ext in task_history:
+        if status == 'PENDING':
+            pending_tasks.add(task)
+            if task not in set_tasks["ever_failed"] and task not in set_tasks["completed"]:
+                if ext:
+                    set_tasks["still_pending_not_ext"].add(task)
+                else:
+                    set_tasks["still_pending_ext"].add(task)
+        elif status == 'DONE':
+            if task in pending_tasks:
+                set_tasks["completed"].add(task)
+            elif task not in set_tasks["completed"]:
+                set_tasks["already_done"].add(task)
+        elif status == 'FAILED':
+            set_tasks["ever_failed"].add(task)
+            if task not in set_tasks["completed"]:
+                set_tasks["failed"].add(task)
+        elif status == 'UNKNOWN':
+            set_tasks["scheduling_error"].add(task)
+
     return set_tasks