diff --git a/db/migrate/20251220070408_add_index_to_messages_yyymm.rb b/db/migrate/20251220070408_add_index_to_messages_yyymm.rb
new file mode 100644
index 0000000..e6b5f9f
--- /dev/null
+++ b/db/migrate/20251220070408_add_index_to_messages_yyymm.rb
@@ -0,0 +1,5 @@
+class AddIndexToMessagesYyymm < ActiveRecord::Migration[8.1]
+  def change
+    add_index :messages, :yyyymm
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 9acb8f4..2c2a6e2 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2025_12_20_051221) do
+ActiveRecord::Schema[8.1].define(version: 2025_12_20_070408) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
   enable_extension "pg_trgm"
@@ -67,6 +67,7 @@
     t.index ["body"], name: "index_messages_on_body", opclass: :gin_trgm_ops, using: :gin
     t.index ["list_id", "list_seq"], name: "index_messages_on_list_id_and_list_seq", unique: true
     t.index ["message_id_header"], name: "index_messages_on_message_id_header"
+    t.index ["yyyymm"], name: "index_messages_on_yyyymm"
   end
 
   add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"