Function remove_const

sql/sql_select.cc:17625–17833

*/

static ORDER *
remove_const(JOIN *join,ORDER *first_order, COND *cond,
             bool change_list, bool *simple_order)
{
  /*
    We can't do ORDER BY using filesort if the select list contains a non
    deterministic value like RAND() or ROWNUM().
    For example:
    SELECT a,ROWNUM() FROM t1 ORDER BY a;

    If we would first sort the table 't1', the ROWNUM() column would be
    generated during end_send() and the order would be wrong.

    Previously we had here also a test of ROLLUP:
    'join->rollup.state == ROLLUP::STATE_NONE'

    I deleted this because the ROLLUP was never enforced because of a
    bug where the inital value of simple_order was ignored.  Having
    ROLLUP tested now when the code is fixed, causes many test failure
    and some wrong results so better to leave the code as it was
    related to ROLLUP.
  */
  *simple_order= !join->select_lex->rownum_in_field_list;
  if (join->only_const_tables())
    return change_list ? 0 : first_order;		// No need to sort

  ORDER *order,**prev_ptr, *tmp_order;
  table_map UNINIT_VAR(first_table); /* protected by first_is_base_table */
  table_map not_const_tables= ~join->const_table_map;
  table_map ref;
  bool first_is_base_table= FALSE;
  DBUG_ENTER("remove_const");
  
  /*
    Join tab is set after make_join_statistics() has been called.
    In case of one table with GROUP BY this function is called before
    join_tab is set for the GROUP_BY expression
  */
  if (join->join_tab)
  {
    if (join->join_tab[join->const_tables].table)
    {
      first_table= join->join_tab[join->const_tables].table->map;
      first_is_base_table= TRUE;
    }
  
    /*
      Cleanup to avoid interference of calls of this function for
      ORDER BY and GROUP BY
    */
    for (JOIN_TAB *tab= join->join_tab + join->const_tables;
         tab < join->join_tab + join->top_join_tab_count;
         tab++)
      tab->cached_eq_ref_table= FALSE;

    JOIN_TAB *head= join->join_tab + join->const_tables;
    *simple_order&= head->on_expr_ref[0] == NULL;
    if (*simple_order && head->table->file->ha_table_flags() & HA_SLOW_RND_POS)