For an EXPLAIN statement, for every SELECT, mysql_select will be called with option SELECT_DESCRIBE.

For main UNION, mysql_explain_union will be called.

For every SELECT in a given union, mysql_explain_union will call mysql_explain_select.

mysql_explain_select will call mysql_select with option SELECT_DESCRIBE.

mysql_select creates a JOIN for select if it does not already exist (it might already exist because if it called for subquery JOIN can be created in JOIN::optimize of outer query when it decided to calculate the value of the subquery). Then it calls JOIN::prepare, JOIN::optimize, JOIN::exec and JOIN::cleanup as usual.

JOIN::exec is called for SELECT with SELECT_DESCRIBE option call select_describe.

select_describe returns the user description of SELECT and calls mysql_explain_union for every inner UNION.

PROBLEM: how it will work with global query optimization?