- Don't call mysql_select() several times for the select that enumerates
  a temporary table with the results of the UNION. Making this call for
  every subquery execution caused O(#enumerated-rows-in-the-outer-query) 
  memory allocations.
- Instead, call join->reinit() and join->exec(), and
  = disable constant table detection for such joins,  
  = provide special handling for table-less constant subqueries.


sql/sql_select.cc:
  BUG#29582: huge memory consumption with union, subselect, joins:
   - Don't mark tables as constant if JOIN::no_const_tables flag is set
sql/sql_select.h:
  BUG#29582: huge memory consumption with union, subselect, joins:
   - Don't mark tables as constant if JOIN::no_const_tables flag is set
sql/sql_union.cc:
  BUG#29582: huge memory consumption with union, subselect, joins:
  - Don't call mysql_select() several times for the select that enumerates
    a temporary table with UNION results. 
  - Instead, call join->reinit() and join->exec().
  - Provide special handling for table-less constant subqueries.