Modify alert group list index (#3502)

# What this PR does

Modifies the database index used by the alert group list page to
accommodate ordering by
[`-started_at`](https://github.com/grafana/oncall/pull/3502/files#diff-68d8b0b2e9b7affe54e5950215e69df4afcc6f7f5dd7eeceb810afda0dd5e1d3R85)

## Which issue(s) this PR fixes

Related to https://github.com/grafana/support-escalations/issues/8567

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

engine/apps/alerts/migrations/0043_remove_alertgroup_alerts_aler_channel_81aeec_idx_and_more.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.2.7 on 2023-12-06 13:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('alerts', '0042_alertgroup_received_at'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='alertgroup',
+            index=models.Index(fields=['channel_id', 'resolved', 'acknowledged', 'silenced', 'root_alert_group_id', 'started_at'], name='alert_group_list_index'),
+        ),
+        migrations.RemoveIndex(
+            model_name='alertgroup',
+            name='alerts_aler_channel_81aeec_idx',
+        ),
+    ]

engine/apps/alerts/models/alert_group.py

@@ -457,7 +457,10 @@ class AlertGroup(AlertGroupSlackRenderingMixin, EscalationSnapshotMixin, models.
             "is_open_for_grouping",
         ]
         indexes = [
-            models.Index(fields=["channel_id", "resolved", "acknowledged", "silenced", "root_alert_group_id"]),
+            models.Index(
+                fields=["channel_id", "resolved", "acknowledged", "silenced", "root_alert_group_id", "started_at"],
+                name="alert_group_list_index",
+            ),
         ]
 
     def __str__(self):

engine/apps/api/views/alert_group.py

@@ -30,7 +30,7 @@ from apps.user_management.models import Team, User
 from common.api_helpers.exceptions import BadRequest
 from common.api_helpers.filters import NO_TEAM_VALUE, DateRangeFilterMixin, ModelFieldFilterMixin
 from common.api_helpers.mixins import PreviewTemplateMixin, PublicPrimaryKeyMixin, TeamFilteringMixin
-from common.api_helpers.paginators import TwentyFiveCursorPaginator
+from common.api_helpers.paginators import AlertGroupCursorPaginator
 
 
 def get_integration_queryset(request):
@@ -309,7 +309,7 @@ class AlertGroupView(
 
     serializer_class = AlertGroupSerializer
 
-    pagination_class = TwentyFiveCursorPaginator
+    pagination_class = AlertGroupCursorPaginator
 
     filter_backends = [SearchFilter, AlertGroupFilterBackend]
     # search_fields = ["=public_primary_key", "=inside_organization_number", "web_title_cache"]

engine/common/api_helpers/paginators.py

@@ -80,6 +80,6 @@ class FifteenPageSizePaginator(PathPrefixedPagePagination):
     page_size = 15
 
 
-class TwentyFiveCursorPaginator(PathPrefixedCursorPagination):
+class AlertGroupCursorPaginator(PathPrefixedCursorPagination):
     page_size = 25
-    ordering = "-started_at"
+    ordering = "-started_at"  # ordering by "-started_at", so it uses the right index (see AlertGroup.Meta.indexes)