diff --git a/db/post_migrate/20250717131454_add_index_to_merge_request_assignees_user_id_and_merge_request_id.rb b/db/post_migrate/20250717131454_add_index_to_merge_request_assignees_user_id_and_merge_request_id.rb
new file mode 100644
index 0000000000000000000000000000000000000000..677162d2df9fabba69642cfe3894c66e8e128fbc
--- /dev/null
+++ b/db/post_migrate/20250717131454_add_index_to_merge_request_assignees_user_id_and_merge_request_id.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddIndexToMergeRequestAssigneesUserIdAndMergeRequestId < Gitlab::Database::Migration[2.3]
+  disable_ddl_transaction!
+
+  milestone '18.3'
+
+  INDEX_NAME = 'index_merge_request_assignees_on_user_id_and_merge_request_id'
+
+  def up
+    add_concurrent_index :merge_request_assignees, [:user_id, :merge_request_id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :merge_request_assignees, INDEX_NAME
+  end
+end
diff --git a/db/schema_migrations/20250717131454 b/db/schema_migrations/20250717131454
new file mode 100644
index 0000000000000000000000000000000000000000..9620d0cb5f5c49c07ebc27655240c68aecb3e65b
--- /dev/null
+++ b/db/schema_migrations/20250717131454
@@ -0,0 +1 @@
+db888e55f012149aa300cb4f6fb80012d9d8e8a8ae925ff3bdfb4ce30bee5ba9
\ No newline at end of file
diff --git a/db/structure.sql b/db/structure.sql
index 5f75a44e60daa3fddb66e8a18664c4f8be9ccc9b..3fffaa4dd159ce9fdb50bfc3070cad6458d92c74 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -36508,6 +36508,8 @@ CREATE INDEX index_merge_request_assignees_on_project_id ON merge_request_assign
 
 CREATE INDEX index_merge_request_assignees_on_user_id ON merge_request_assignees USING btree (user_id);
 
+CREATE INDEX index_merge_request_assignees_on_user_id_and_merge_request_id ON merge_request_assignees USING btree (user_id, merge_request_id);
+
 CREATE INDEX index_merge_request_assignment_events_on_project_id ON merge_request_assignment_events USING btree (project_id);
 
 CREATE INDEX index_merge_request_assignment_events_on_user_id ON merge_request_assignment_events USING btree (user_id);