audit-logs: Add index to RealmAuditLog for realm and event type.

Adds an index on RealmAuditLog for the realm, event_type, and
event_time in order to improve database queries on these audit logs.

tabbott verified using EXPLAIN ANALYZE that this also considerably
speeds up queries that order by ID rather than event_time, but
event_time is how these should be ordered given the possibility of
backfills.

zerver/migrations/0510_add_realmauditlog_realm_event_type_index.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.12 on 2024-04-17 10:33
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0509_fix_emoji_metadata"),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name="realmauditlog",
+            index=models.Index(
+                fields=["realm", "event_type", "event_time"], name="zerver_realmauditlog_realm__event_type__event_time"
+            ),
+        ),
+    ]

zerver/models/realm_audit_logs.py

@@ -227,6 +227,10 @@ class RealmAuditLog(AbstractRealmAuditLog):
 
     class Meta:
         indexes = [
+            models.Index(
+                name="zerver_realmauditlog_realm__event_type__event_time",
+                fields=["realm", "event_type", "event_time"],
+            ),
             models.Index(
                 name="zerver_realmauditlog_user_subscriptions_idx",
                 fields=["modified_user", "modified_stream"],
@@ -237,7 +241,7 @@ class Meta:
                         AbstractRealmAuditLog.SUBSCRIPTION_DEACTIVATED,
                     ]
                 ),
-            )
+            ),
         ]
 
     @override