Optimize HostList API: conditional DISTINCT + composite index on JobHostSummary (#16530)

AAP-81517 — Optimize HostList API: conditional DISTINCT + composite index

Make .distinct() conditional on host_filter being set — without it the
RBAC IN subquery on a direct FK cannot produce duplicates, so DISTINCT
is pure overhead. Add composite index (host_id, id DESC) on
main_jobhostsummary so the with_latest_summary_id() correlated subquery
can use an index-only top-1 scan instead of scanning and sorting.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

awx/api/views/__init__.py

@@ -1926,7 +1926,8 @@ class HostList(HostRelatedSearchMixin, ListCreateAPIView):
         if filter_string:
             filter_qs = SmartFilter.query_from_string(filter_string)
             qs &= filter_qs
-        return qs.distinct().with_latest_summary_id()
+            qs = qs.distinct()
+        return qs.with_latest_summary_id()
 
     def list(self, *args, **kwargs):
         try:

awx/main/migrations/0206_jobhostsummary_host_id_idx.py

@@ -0,0 +1,17 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('main', '0205_add_ordering_to_instancegroup_and_workflow_nodes'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='jobhostsummary',
+            index=models.Index(
+                fields=['host', '-id'],
+                name='main_jobhostsumm_host_id_desc',
+            ),
+        ),
+    ]

awx/main/models/jobs.py

@@ -1092,6 +1092,9 @@ class JobHostSummary(CreatedModifiedModel):
         unique_together = [('job', 'host_name')]
         verbose_name_plural = _('job host summaries')
         ordering = ('-pk',)
+        indexes = [
+            models.Index(fields=['host', '-id'], name='main_jobhostsumm_host_id_desc'),
+        ]
 
     job = models.ForeignKey(
         'Job',