perf: aggregate batch status statistics in a single query

backend/tests/test_optimization.py

@@ -249,6 +249,38 @@ def test_last_review_updates_batch_and_keeps_zero(sqlite_db):
         assert sheet["D2"].value == 0
 
 
+@pytest.mark.parametrize("batch_count", [0, 20])
+def test_batch_list_status_statistics_use_constant_query_count(sqlite_db, batch_count):
+    from wisegrade.api import list_batches
+    db = sqlite_db
+    item = project(db)
+    expected = {}
+    states = ["scored", "reviewed", "needs_review", "failed", "unscorable"]
+    for index in range(batch_count):
+        created = services.create_batch(SimpleNamespace(course_version_id=1, rubric_version_id=1), db)
+        batch = db.get(ScoringBatch, created["id"])
+        criterion = batch.rubric_snapshot["criteria"][0]["criterion_id"]
+        state = states[index % len(states)]
+        db.add(CriterionScore(batch_id=batch.id, project_id=item.id, criterion_id=criterion, status=state))
+        expected[batch.id] = {state: 1}
+    db.commit()
+    queries = []
+    def trace(_conn, _cursor, statement, _parameters, _context, _many):
+        if statement.lstrip().upper().startswith("SELECT"):
+            queries.append(statement)
+    event.listen(db.get_bind(), "before_cursor_execute", trace)
+    try:
+        listed = list_batches(db)
+    finally:
+        event.remove(db.get_bind(), "before_cursor_execute", trace)
+    assert len(queries) <= (2 if batch_count else 1), len(queries)
+    assert [row["id"] for row in listed] == sorted(expected, reverse=True)
+    for row in listed:
+        assert row["status_counts"] == expected[row["id"]]
+        for state in ("needs_review", "failed", "unscorable"):
+            assert row[state] == expected[row["id"]].get(state, 0)
+
+
 def test_pdf_html_preserves_zero_and_none():
     batch = SimpleNamespace(id=1, course_version_id=1, rubric_version_id=1, model_config_snapshot={},
                             embedding_model_name="test", embedding_model_version="test")