diff --git a/backend/src/main/resources/db/migration/V12__add_guest_order_columns.sql b/backend/src/main/resources/db/migration/V12__add_guest_order_columns.sql
index f508fed..0b4412e 100644
--- a/backend/src/main/resources/db/migration/V12__add_guest_order_columns.sql
+++ b/backend/src/main/resources/db/migration/V12__add_guest_order_columns.sql
@@ -1,24 +1,25 @@
 -- Allows orders without a registered user (guest checkout).
 -- Users can place and pay for letters without creating an account.
 --
--- user_id:  previously NOT NULL — drop the constraint so guest orders
+-- user_id:  previously NOT NULL - drop the constraint so guest orders
 --           can be created without a registered user. The FK stays in
 --           place (NULL user_id is FK-legal).
 -- guest_email: contact address for the guest. Used to send the magic
 --              link that lets them revisit their order status.
--- guest_token: opaque UUID v4 — the only credential a guest has. Acts
+-- guest_token: opaque UUID v4 - the only credential a guest has. Acts
 --               as their session token for order lookup + payment confirm.
 
 ALTER TABLE orders ALTER COLUMN user_id DROP NOT NULL;
 ALTER TABLE orders ADD COLUMN guest_email VARCHAR(255);
 ALTER TABLE orders ADD COLUMN guest_token UUID;
 
--- Partial unique index: only enforce uniqueness on non-NULL tokens.
--- Multiple NULLs allowed — existing user-owned orders have no token,
--- and that's fine.
+-- Unique index on guest_token. Both H2 (tests/dev) and PostgreSQL (prod)
+-- treat NULLs as distinct in a UNIQUE index, so user-owned orders (which
+-- have a NULL token) never collide, while non-NULL guest tokens are
+-- enforced unique. A plain index is used instead of a partial
+-- (WHERE guest_token IS NOT NULL) index because H2 does not support
+-- partial indexes, and the plain form preserves the intended semantics.
 CREATE UNIQUE INDEX idx_orders_guest_token
-    ON orders(guest_token)
-    WHERE guest_token IS NOT NULL;
+    ON orders(guest_token);
 CREATE INDEX idx_orders_guest_email
-    ON orders(guest_email)
-    WHERE guest_email IS NOT NULL;
+    ON orders(guest_email);