SHOW EXPLAIN allows one to view EXPLAIN of a currently executing query.
However, it doesn't handle this use case:
A really huge query runs for a long time and never finishes. Since it never finishes, one cannot view ANALYZE output. Adding "LIMIT N" to the top query is of limited help.
One can view the EXPLAIN, but the query has a lot of subqueries that have many-table joins. One can see multiple sub-optimal access methods, and it is not clear which of them are the ones that are causing the performance problem.
Suggested solution: extend SHOW EXPLAIN to report SHOW ANALYZE FORMAT=JSON data of the query that's currently running.