Function select_describe

sql/sql_select.cc:31821–31864

*/ 

static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
			    bool distinct,const char *message)
{
  THD *thd=join->thd;
  DBUG_ENTER("select_describe");

  if (join->select_lex->pushdown_select)
  {
    /*
      The whole statement was pushed down to a Smart Storage Engine. Do not
      attempt to produce a query plan locally.
    */
    DBUG_VOID_RETURN;
  }

  /* Update the QPF with latest values of using_temporary, using_filesort */
  for (SELECT_LEX_UNIT *unit= join->select_lex->first_inner_unit();
       unit;
       unit= unit->next_unit())
  {
    /*
      This fix_fields() call is to handle an edge case like this:
       
        SELECT ... UNION SELECT ... ORDER BY (SELECT ...)
      
      for such queries, we'll get here before having called
      subquery_expr->fix_fields(), which will cause failure to
    */
    if (unit->item && !unit->item->fixed())
    {
      Item *ref= unit->item;
      if (unit->item->fix_fields(thd, &ref))
        DBUG_VOID_RETURN;
      DBUG_ASSERT(ref == unit->item);
    }

    if (unit->explainable())
    {
      if (mysql_explain_union(thd, unit, unit->result))
        DBUG_VOID_RETURN;
    }
  }
  DBUG_VOID_RETURN;
}


bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)