soft_reactivation: Add a partial index to speed up event lookups.

The full auditlog table is moderately large, and the previously-chosen
index (on `modified_user_id`) is not terribly specific.

zerver/lib/soft_deactivation.py

@@ -163,6 +163,10 @@ def add_missing_messages(user_profile: UserProfile) -> None:
     # For stream messages we need to check messages against data from
     # RealmAuditLog for visibility to user. So we fetch the subscription logs.
     stream_ids = [sub["recipient__type_id"] for sub in all_stream_subs]
+
+    # We have a partial index on RealmAuditLog for these rows -- if
+    # this set changes, the partial index must be updated as well, to
+    # keep this query performant
     events = [
         RealmAuditLog.SUBSCRIPTION_CREATED,
         RealmAuditLog.SUBSCRIPTION_DEACTIVATED,

zerver/migrations/0446_realmauditlog_zerver_realmauditlog_user_subscriptions_idx.py

@@ -0,0 +1,26 @@
+# Generated by Django 4.2 on 2023-04-28 19:06
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0445_drop_userpresenceold"),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name="realmauditlog",
+            index=models.Index(
+                name="zerver_realmauditlog_user_subscriptions_idx",
+                fields=["modified_user", "modified_stream"],
+                condition=models.Q(
+                    event_type__in=[
+                        301,  # RealmAuditLog.SUBSCRIPTION_CREATED
+                        302,  # RealmAuditLog.SUBSCRIPTION_ACTIVATED
+                        303,  # RealmAuditLog.SUBSCRIPTION_DEACTIVATED
+                    ]
+                ),
+            ),
+        ),
+    ]

zerver/models.py

@@ -4496,6 +4496,21 @@ class RealmAuditLog(AbstractRealmAuditLog):
             return f"{self.modified_stream!r} {self.event_type} {self.event_time} {self.id}"
         return f"{self.realm!r} {self.event_type} {self.event_time} {self.id}"
 
+    class Meta:
+        indexes = [
+            models.Index(
+                name="zerver_realmauditlog_user_subscriptions_idx",
+                fields=["modified_user", "modified_stream"],
+                condition=Q(
+                    event_type__in=[
+                        AbstractRealmAuditLog.SUBSCRIPTION_CREATED,
+                        AbstractRealmAuditLog.SUBSCRIPTION_ACTIVATED,
+                        AbstractRealmAuditLog.SUBSCRIPTION_DEACTIVATED,
+                    ]
+                ),
+            )
+        ]
+
 
 class UserHotspot(models.Model):
     user = models.ForeignKey(UserProfile, on_delete=CASCADE)