Fix n+1 query to fetch tags in the dags list page

airflow-core/src/airflow/api_fastapi/common/db/dags.py

@@ -20,6 +20,7 @@
 from typing import TYPE_CHECKING
 
 from sqlalchemy import func, select
+from sqlalchemy.orm import selectinload
 
 from airflow.api_fastapi.common.db.common import (
     apply_filters_to_select,
@@ -33,7 +34,7 @@
 
 
 def generate_dag_with_latest_run_query(max_run_filters: list[BaseParam], order_by: SortParam) -> Select:
-    query = select(DagModel)
+    query = select(DagModel).options(selectinload(DagModel.tags))
 
     max_run_id_query = (  # ordering by id will not always be "latest run", but it's a simplifying assumption
         select(DagRun.dag_id, func.max(DagRun.id).label("max_dag_run_id"))

airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dags.py

@@ -31,6 +31,7 @@
 from airflow.utils.state import DagRunState, TaskInstanceState
 from airflow.utils.types import DagRunTriggeredByType, DagRunType
 
+from tests_common.test_utils.asserts import count_queries
 from tests_common.test_utils.db import (
     clear_db_assets,
     clear_db_connections,
@@ -524,6 +525,71 @@ def test_get_dags_filter_has_import_errors(self, session, test_client, filter_va
         assert body["total_entries"] == 1
         assert [dag["dag_id"] for dag in body["dags"]] == expected_ids
 
+    def test_get_dags_no_n_plus_one_queries(self, session, test_client):
+        """Test that fetching DAGs with tags doesn't trigger n+1 queries."""
+        num_dags = 5
+        for i in range(num_dags):
+            dag_id = f"test_dag_queries_{i}"
+            dag_model = DagModel(
+                dag_id=dag_id,
+                bundle_name="dag_maker",
+                fileloc=f"/tmp/{dag_id}.py",
+                is_stale=False,
+            )
+            session.add(dag_model)
+            session.flush()
+
+            for j in range(3):
+                tag = DagTag(name=f"tag_{i}_{j}", dag_id=dag_id)
+                session.add(tag)
+
+        session.commit()
+        session.expire_all()
+
+        with count_queries() as result:
+            response = test_client.get("/dags", params={"limit": 10})
+
+        assert response.status_code == 200
+        body = response.json()
+        dags_with_our_prefix = [d for d in body["dags"] if d["dag_id"].startswith("test_dag_queries_")]
+        assert len(dags_with_our_prefix) == num_dags
+        for dag in dags_with_our_prefix:
+            assert len(dag["tags"]) == 3
+
+        first_query_count = sum(result.values())
+
+        # Add more DAGs and verify query count doesn't scale linearly
+        for i in range(num_dags, num_dags + 3):
+            dag_id = f"test_dag_queries_{i}"
+            dag_model = DagModel(
+                dag_id=dag_id,
+                bundle_name="dag_maker",
+                fileloc=f"/tmp/{dag_id}.py",
+                is_stale=False,
+            )
+            session.add(dag_model)
+            session.flush()
+
+            for j in range(3):
+                tag = DagTag(name=f"tag_{i}_{j}", dag_id=dag_id)
+                session.add(tag)
+
+        session.commit()
+        session.expire_all()
+
+        with count_queries() as result2:
+            response = test_client.get("/dags", params={"limit": 15})
+
+        assert response.status_code == 200
+        second_query_count = sum(result2.values())
+
+        # With n+1, adding 3 DAGs would add ~3 tag queries
+        # Without n+1, query count should remain nearly identical
+        assert second_query_count - first_query_count < 3, (
+            f"Added 3 DAGs but query count increased by {second_query_count - first_query_count} "
+            f"({first_query_count} → {second_query_count}), suggesting n+1 queries for tags"
+        )
+
 
 class TestPatchDag(TestDagEndpoint):
     """Unit tests for Patch DAG."""

airflow-core/tests/unit/api_fastapi/core_api/routes/ui/test_dags.py

@@ -25,13 +25,15 @@
 from sqlalchemy.orm import Session
 
 from airflow.models import DagRun
+from airflow.models.dag import DagModel, DagTag
 from airflow.models.dag_favorite import DagFavorite
 from airflow.models.hitl import HITLDetail
 from airflow.sdk.timezone import utcnow
 from airflow.utils.session import provide_session
 from airflow.utils.state import DagRunState, TaskInstanceState
 from airflow.utils.types import DagRunTriggeredByType, DagRunType
 
+from tests_common.test_utils.asserts import count_queries
 from unit.api_fastapi.core_api.routes.public.test_dags import (
     DAG1_ID,
     DAG2_ID,
@@ -231,6 +233,71 @@ def test_should_response_403(self, unauthorized_test_client):
         response = unauthorized_test_client.get("/dags", params={})
         assert response.status_code == 403
 
+    def test_get_dags_no_n_plus_one_queries(self, session, test_client):
+        """Test that fetching DAGs with tags doesn't trigger n+1 queries."""
+        num_dags = 5
+        for i in range(num_dags):
+            dag_id = f"test_dag_queries_ui_{i}"
+            dag_model = DagModel(
+                dag_id=dag_id,
+                bundle_name="dag_maker",
+                fileloc=f"/tmp/{dag_id}.py",
+                is_stale=False,
+            )
+            session.add(dag_model)
+            session.flush()
+
+            for j in range(3):
+                tag = DagTag(name=f"tag_ui_{i}_{j}", dag_id=dag_id)
+                session.add(tag)
+
+        session.commit()
+        session.expire_all()
+
+        with count_queries() as result:
+            response = test_client.get("/dags", params={"limit": 10})
+
+        assert response.status_code == 200
+        body = response.json()
+        dags_with_our_prefix = [d for d in body["dags"] if d["dag_id"].startswith("test_dag_queries_ui_")]
+        assert len(dags_with_our_prefix) == num_dags
+        for dag in dags_with_our_prefix:
+            assert len(dag["tags"]) == 3
+
+        first_query_count = sum(result.values())
+
+        # Add more DAGs and verify query count doesn't scale linearly
+        for i in range(num_dags, num_dags + 3):
+            dag_id = f"test_dag_queries_ui_{i}"
+            dag_model = DagModel(
+                dag_id=dag_id,
+                bundle_name="dag_maker",
+                fileloc=f"/tmp/{dag_id}.py",
+                is_stale=False,
+            )
+            session.add(dag_model)
+            session.flush()
+
+            for j in range(3):
+                tag = DagTag(name=f"tag_ui_{i}_{j}", dag_id=dag_id)
+                session.add(tag)
+
+        session.commit()
+        session.expire_all()
+
+        with count_queries() as result2:
+            response = test_client.get("/dags", params={"limit": 15})
+
+        assert response.status_code == 200
+        second_query_count = sum(result2.values())
+
+        # With n+1, adding 3 DAGs would add ~3 tag queries
+        # Without n+1, query count should remain nearly identical
+        assert second_query_count - first_query_count < 3, (
+            f"Added 3 DAGs but query count increased by {second_query_count - first_query_count} "
+            f"({first_query_count} → {second_query_count}), suggesting n+1 queries for tags"
+        )
+
     @pytest.mark.usefixtures("configure_git_connection_for_dag_bundle")
     def test_latest_run_should_return_200(self, test_client):
         response = test_client.get(f"/dags/{DAG1_ID}/latest_run")