zerver: Add a partial audit log index for counting active users.

This index is used by `active_users_audit:is_bot:day`, and provides
roughly a 2x speedup.  The existing
`zerver_realmauditlog_realm__event_type__event_time` is used if there
is a realm limit, but the standard statistics fill runs for all realms
at once, and thus cannot use it.

zerver/migrations/0528_realmauditlog_zerver_realmauditlog_user_activations_idx.py

@@ -0,0 +1,26 @@
+from django.contrib.postgres.operations import AddIndexConcurrently
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    dependencies = [
+        ("zerver", "0527_presencesequence"),
+    ]
+
+    operations = [
+        AddIndexConcurrently(
+            model_name="realmauditlog",
+            index=models.Index(
+                # event_type__in:
+                #   AbstractRealmAuditLog.USER_CREATED,
+                #   AbstractRealmAuditLog.USER_ACTIVATED,
+                #   AbstractRealmAuditLog.USER_DEACTIVATED,
+                #   AbstractRealmAuditLog.USER_REACTIVATED,
+                condition=models.Q(("event_type__in", [101, 102, 103, 104])),
+                fields=["modified_user", "event_time"],
+                name="zerver_realmauditlog_user_activations_idx",
+            ),
+        ),
+    ]

zerver/models/realm_audit_logs.py

@@ -242,6 +242,19 @@ class RealmAuditLog(AbstractRealmAuditLog):
                     ]
                 ),
             ),
+            models.Index(
+                # Used in analytics/lib/counts.py for computing active users for realm_active_humans
+                name="zerver_realmauditlog_user_activations_idx",
+                fields=["modified_user", "event_time"],
+                condition=Q(
+                    event_type__in=[
+                        AbstractRealmAuditLog.USER_CREATED,
+                        AbstractRealmAuditLog.USER_ACTIVATED,
+                        AbstractRealmAuditLog.USER_DEACTIVATED,
+                        AbstractRealmAuditLog.USER_REACTIVATED,
+                    ]
+                ),
+            ),
         ]
 
     @override