models: Add a partial index on unread DMs.

This helps the find_first_unread_anchor call for DMs with a user.

zerver/migrations/0742_usermessage_zerver_usermessage_is_private_unread_message_id.py

@@ -0,0 +1,22 @@
+from django.contrib.postgres.operations import AddIndexConcurrently
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    atomic = False
+    dependencies = [
+        ("zerver", "0741_pushdevice_zerver_pushdevice_user_bouncer_device_id_idx"),
+    ]
+
+    operations = [
+        AddIndexConcurrently(
+            model_name="usermessage",
+            index=models.Index(
+                models.F("user_profile"),
+                models.F("message"),
+                condition=models.Q(("flags__andnz", 2048), ("flags__andz", 1)),
+                name="zerver_usermessage_is_private_unread_message_id",
+            ),
+        ),
+        migrations.RunSQL("ANALYZE zerver_usermessage"),
+    ]

zerver/models/messages.py

@@ -636,6 +636,15 @@ class UserMessage(AbstractUserMessage):
                 condition=Q(flags__andnz=AbstractUserMessage.flags.is_private.mask),
                 name="zerver_usermessage_is_private_message_id",
             ),
+            models.Index(
+                "user_profile",
+                "message",
+                condition=(
+                    Q(flags__andnz=AbstractUserMessage.flags.is_private.mask)
+                    & Q(flags__andz=AbstractUserMessage.flags.read.mask)
+                ),
+                name="zerver_usermessage_is_private_unread_message_id",
+            ),
             models.Index(
                 "user_profile",
                 "message",