- After the merge from mysql-5.6, open_tables() did not call
  open_and_process_table() for temporary table. The logic was that temporary
  tables were already opened when mysql_execute_command() has called 
  open_temporary_tables().
  This worked for the most part, except for temporary tables of type MERGE. for
  which open_and_process_table() must call 
    table->file->extra(HA_EXTRA_ADD_CHILDREN_LIST).
  Failure to make this call resulted in crash further in execution.

  tables->table->file->extra(HA_EXTRA_ADD_CHILDREN_LIST)