MySQL: does Index Merge Sort-Union access algorithm use temporary table?
I've performed an experiment: rewrote the query from using 9 ORs in a single query to a UNION DISTINCT of 9 subqueries. As expected, EXPLAIN shows for a rewritten query in the last row: select_type = "UNION RESULT", Extra = "Using temporary". Such change was done to have a query that surely uses temporary table to compare with.

Now, when I launched an app using this query in its original form (9 ORs) - each check Created_tmp_tables increased only by 1 (from the manual: invocation of the SHOW STATUS increments the global Created_tmp_tables value by 1).

When I launched the app using rewritten query Created_tmp_tables grows very fast.

I think this experiment is enough to show that temporary tables are not being created in the case of index_merge Sort-Union.

Conclusion: no, it does not.