In other RDBMS, there is the concept of GLOBAL or LOCAL indexes on partitioned tables.
MariaDB currently has LOCAL indexes, which are local to each partition in a table.
A GLOBAL index on the other hand is a one-to-many relationship, allowing one index partition to map to many table partitions and avoid the scanning behavior when partition keys are not in a query.
Implementations on other databases:
Good justification on why postgres (and MariaDB) should have it: https://www.postgresql.org/message-id/CALtqXTcurqy1PKXzP9XO%3DofLLA5wBSo77BnUnYVEZpmcA3V0ag%40mail.gmail.com