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 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

@@ -2416,7 +2416,7 @@ make_join_statistics(JOIN *join, TABLE_LIST *tables, COND *conds,
 
     if ((table->s->system || table->file->records <= 1) && ! s->dependent &&
 	!(table->file->table_flags() & HA_NOT_EXACT_COUNT) &&
-        !table->fulltext_searched)
+        !table->fulltext_searched && !join->no_const_tables)
     {
       set_position(join,const_count++,s,(KEYUSE*) 0);
     }