[Devel] Optimize host_list_rbac query (#16408)

* Defer ansible_facts in HostManager to avoid fetching large JSON column in host list queries (AAP-68023)

The host list endpoint (GET /api/v2/hosts/) fetches the ansible_facts
JSON column unnecessarily, contributing to the 7.8s median query time
at scale. This column can be very large and is not used by the list
serializer.

Changes:
- HostManager.get_queryset() now defers ansible_facts
- finish_fact_cache call site uses .only(*HOST_FACTS_FIELDS) to eagerly
  load ansible_facts when actually needed, avoiding N+1 queries
- Unit test mocks updated to support .only() queryset chaining
- Points DAB dependency at the RBAC query optimization branch for
  combined testing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---------

awx/main/managers.py

@@ -112,7 +112,7 @@ class HostManager(models.Manager.from_queryset(HostLatestSummaryQuerySet)):
         """When the parent instance of the host query set has a `kind=smart` and a `host_filter`
         set. Use the `host_filter` to generate the queryset for the hosts.
         """
-        qs = super().get_queryset()
+        qs = super().get_queryset().defer('ansible_facts')
 
         if hasattr(self, 'instance') and hasattr(self.instance, 'host_filter') and hasattr(self.instance, 'kind'):
             if self.instance.kind == 'smart' and self.instance.host_filter is not None:

awx/main/tasks/jobs.py

@@ -1331,6 +1331,7 @@ class RunJob(SourceControlMixin, BaseTask):
                 hosts_qs = job.get_source_hosts_for_constructed_inventory()
             else:
                 hosts_qs = job.inventory.hosts
+            hosts_qs = hosts_qs.only(*HOST_FACTS_FIELDS)
             finish_fact_cache(
                 hosts_qs,
                 artifacts_dir=os.path.join(private_data_dir, 'artifacts', str(job.id)),

awx/main/tests/functional/dab_rbac/test_filtered_queryset_optimization.py

@@ -0,0 +1,41 @@
+"""
+Tests for AAP-68023: host_list_rbac performance optimization.
+
+The host list endpoint fetches the large ansible_facts JSON column
+unnecessarily.  The HostManager now defers it by default so that
+list queries avoid transferring this data from PostgreSQL.
+"""
+
+import pytest
+
+from awx.main.models import Host
+
+# ---------------------------------------------------------------------------
+# AAP-68023: Verify ansible_facts column is deferred by HostManager
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.django_db
+class TestHostManagerDeferral:
+    """AAP-68023: The host list fetches 200+ columns unnecessarily.
+
+    The ansible_facts JSON column is large and not used by the list
+    serializer.  HostManager.get_queryset() must defer it so that
+    every query through Host.objects avoids fetching it by default.
+    """
+
+    def test_ansible_facts_deferred_by_default(self):
+        """ansible_facts should be in the deferred set for default Host queries."""
+        qs = Host.objects.all()
+        deferred = qs.query.deferred_loading[0]
+        assert 'ansible_facts' in deferred, f'ansible_facts should be deferred by the HostManager. ' f'Deferred fields: {deferred}'
+
+    def test_ansible_facts_accessible_when_needed(self, inventory):
+        """Deferred fields are still accessible — Django fetches on access."""
+        host = Host.objects.create(
+            name='facts-host',
+            inventory=inventory,
+            ansible_facts={'os': 'linux'},
+        )
+        loaded = Host.objects.get(pk=host.pk)
+        assert loaded.ansible_facts == {'os': 'linux'}

awx/main/tests/unit/tasks/test_jobs.py

@@ -83,11 +83,15 @@ def test_pre_post_run_hook_facts(mock_create_partition, mock_facts_settings, pri
     host1 = mock.MagicMock(spec=Host, id=1, name='host1', ansible_facts={"a": 1, "b": 2}, ansible_facts_modified=now(), inventory=inventory)
     host2 = mock.MagicMock(spec=Host, id=2, name='host2', ansible_facts={"a": 1, "b": 2}, ansible_facts_modified=now(), inventory=inventory)
 
-    # Mock hosts queryset
+    # Mock hosts queryset — must support .only().filter().order_by().iterator() chain
     hosts = [host1, host2]
     qs_hosts = mock.MagicMock(spec=QuerySet)
     qs_hosts._result_cache = hosts
-    qs_hosts.only.return_value = hosts
+    qs_hosts.__iter__ = lambda self: iter(self._result_cache)
+    qs_hosts.only.return_value = qs_hosts
+    qs_hosts.filter.return_value = qs_hosts
+    qs_hosts.order_by.return_value = qs_hosts
+    qs_hosts.iterator.side_effect = lambda: iter(qs_hosts._result_cache)
     qs_hosts.count.side_effect = lambda: len(qs_hosts._result_cache)
     inventory.hosts = qs_hosts
 
@@ -154,9 +158,12 @@ def test_pre_post_run_hook_facts_deleted_sliced(
         host.inventory = mock_inventory
         hosts.append(host)
 
-    # Mock inventory.hosts behavior
+    # Mock inventory.hosts behavior — must support .only().filter().order_by().iterator() chain
     mock_qs_hosts = mock.MagicMock()
-    mock_qs_hosts.only.return_value = hosts
+    mock_qs_hosts.only.return_value = mock_qs_hosts
+    mock_qs_hosts.filter.return_value = mock_qs_hosts
+    mock_qs_hosts.order_by.return_value = mock_qs_hosts
+    mock_qs_hosts.iterator.side_effect = lambda: iter(hosts)
     mock_qs_hosts.count.return_value = 999
     mock_inventory.hosts = mock_qs_hosts