Indexing a reference column
On the MySQL support page they write some about index optimization. (I assume MariaDB isn't deviating too much from this since they're somewhat similar.) Here they write that indexes are not so important on small tables.


  Indexes are less important for queries on small tables, or big tables
  where report queries process most or all of the rows. When a query
  needs to access most of the rows, reading sequentially is faster than
  working through an index. Sequential reads minimize disk seeks, even
  if not all the rows are needed for the query.


On the page about How to avoid full table scans they define the limit as "more than a few rows". 


  For tables with more than a few rows, consider redesigning the query
  by adding an index for one or more of the columns tested in the WHERE
  clause.


I personally would add an index just because I always add index to the key on all tables. In your case I don't know if the index would contribute so much though.