WHERE ONE OF SELECT* IS DISTINCT FAILS.
ISSUE:
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There are 2 issues related to explain union.
1. If we have subquery with union of selects. And, one of
   the select need temp table to materialize its results
   then it will replace its query structure with a simple
   select from temporary table. Trying to display new
   internal temporary table scan resulted in crash. But to
   display the query plan, we should save the original
   query structure.
2. Multiple execution of prepared explain statement which
   have union of subqueries resulted in crash. If we have
   constant subqueries, fake select used in union operation
   will be evaluated once before using it for explain.
   During first execution we have set fake select options to
   SELECT_DESCRIBE, but did not reset after the explain.
   Hence during next execution of prepared statement during
   first time evaluation of fake select we had our select
   options as SELECT_DESCRIBE this resulted in improperly
   initialized data structures and crash.

SOLUTION:
---------
1. If called by explain now we save the original query
   structure. And this will be used for displaying.
2. Reset the fake select options after it is called for
   explain of union.

sql/sql_select.cc:
  Reset the fake select options after it is called for explain of union
sql/sql_union.cc:
  If called by explain but not from select_describe and we
  need a temp table, then we create a temp join to preserve
  original query structure.