Update alert group search to force index in MySQL (#4731)

Related to grafana/oncall-private#2679

Confirmed the query rewriting works via MySQL query logs (index is also
forced for the stats queries):

`2024-07-24T19:50:59.482751Z 3977 Query SELECT `alerts_alertgroup`.`id`
FROM `alerts_alertgroup` FORCE INDEX (`alert_group_list_index`) WHERE
(`alerts_alertgroup`.`channel_id` IN (13) AND (1) AND
(`alerts_alertgroup`.`public_primary_key` LIKE 'test' OR
`alerts_alertgroup`.`inside_organization_number` LIKE 'test' OR
`alerts_alertgroup`.`web_title_cache` LIKE '%test%') AND
`alerts_alertgroup`.`root_alert_group_id` IS NULL AND
((`alerts_alertgroup`.`silenced` = ('0') AND
`alerts_alertgroup`.`acknowledged` = ('0') AND
`alerts_alertgroup`.`resolved` = ('0')) OR
(`alerts_alertgroup`.`acknowledged` = ('1') AND
`alerts_alertgroup`.`resolved` = ('0'))) AND
`alerts_alertgroup`.`started_at` >= '2024-06-24 19:50:58' AND
`alerts_alertgroup`.`started_at` <= '2024-07-24 19:50:58') ORDER BY
`alerts_alertgroup`.`started_at` DESC LIMIT 26
`

Rewriting will only be applied to the alert group search queries, when
the feature flags are enabled. Dependency was already listed as a
requirement.

Author: matiasb

engine/apps/api/views/alert_group.py

@@ -335,9 +335,10 @@ def get_queryset(self, ignore_filtering_by_available_teams=False):
         if settings.ALERT_GROUPS_DISABLE_PREFER_ORDERING_INDEX:
             # workaround related to MySQL "ORDER BY LIMIT Query Optimizer Bug"
             # read more: https://hackmysql.com/infamous-order-by-limit-query-optimizer-bug/
-            # this achieves the same effect as "FORCE INDEX (alert_group_list_index)" when
-            # paired with "ORDER BY started_at_optimized DESC" (ordering is performed in AlertGroupCursorPaginator).
-            queryset = queryset.extra({"started_at_optimized": "alerts_alertgroup.started_at + 0"})
+            from django_mysql.models import add_QuerySetMixin
+
+            queryset = add_QuerySetMixin(queryset)
+            queryset = queryset.force_index("alert_group_list_index")
 
         # Filter by labels. Since alert group labels are "static" filter by names, not IDs.
         label_query = self.request.query_params.getlist("label", [])

engine/common/api_helpers/paginators.py

@@ -1,6 +1,5 @@
 import typing
 
-from django.conf import settings
 from rest_framework.pagination import BasePagination, CursorPagination, PageNumberPagination
 from rest_framework.response import Response
 
@@ -86,4 +85,4 @@ class FifteenPageSizePaginator(PathPrefixedPagePagination):
 
 class AlertGroupCursorPaginator(PathPrefixedCursorPagination):
     page_size = 25
-    ordering = "-started_at_optimized" if settings.ALERT_GROUPS_DISABLE_PREFER_ORDERING_INDEX else "-started_at"
+    ordering = "-started_at"

engine/settings/base.py

@@ -189,6 +189,8 @@ class DatabaseTypes:
 
     pymysql.install_as_MySQLdb()
 
+DJANGO_MYSQL_REWRITE_QUERIES = True
+
 ALERT_GROUPS_DISABLE_PREFER_ORDERING_INDEX = DATABASE_TYPE == DatabaseTypes.MYSQL and getenv_boolean(
     "ALERT_GROUPS_DISABLE_PREFER_ORDERING_INDEX", default=False
 )
@@ -293,6 +295,9 @@ class DatabaseTypes:
     "apps.chatops_proxy",
 ]
 
+if DATABASE_TYPE == DatabaseTypes.MYSQL:
+    INSTALLED_APPS += ["django_mysql"]
+
 REST_FRAMEWORK = {
     "DEFAULT_PARSER_CLASSES": (
         "rest_framework.parsers.JSONParser",