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  1. MariaDB Server
  2. MDEV-23825

Join select_handler and Pushdown_select + XPand changes

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      Description

      Under terms of this task we'll do two things:

      • Join classes select_handler and Pushdown_select into a single class
        The current implementation with two parallel classes (select_handler and Pushdown_select) is hard to follow. Non of them can work without the other. The two-step construction stage looks too complex:

          select_lex->select_h= select_lex->find_select_handler(thd);
          if (select_lex->select_h)
          {
            /* Create a Pushdown_select object for later execution of the query */
            if (!(select_lex->pushdown_select=
              new (thd->mem_root) Pushdown_select(select_lex,
                                                  select_lex->select_h)))
            {
              delete select_lex->select_h;
              select_lex->select_h= NULL;
              DBUG_RETURN(TRUE);
            }
          }
        

        With a single object it will be as simple as:

          select_lex->pushdown_select= select_lex->find_select_handler(thd);
        

        The destruction stage looks dangerous in the current implementation:

        Pushdown_select::~Pushdown_select()
        {
         if (handler->table)
           free_tmp_table(handler->thd, handler->table);
         delete handler;
         select->select_h= NULL;
        }
        

        So the server deletes a Pushdown_select instance, which then deletes the select_handler. It's much safer just to delete a single object.

      • Make it possible for the engine (e.g. XPand) to create a TABLE on its own and then reuse it during select_handler() creation time by assigning to select_handler::table. This is needed to avoid two executions of create_tmp_table() which currenly happen with XPand. So during the preparation stage, the TABLE will already be assigned to select_handler::table and select_handler will not need to make it again during the preparation stage. The method will looks about like this:

        bool select_handler::prepare()
        {
          /*
            Some engines (e.g. XPand) initialize "table" on their own.
            So we need to create a temporary table only if "table" is NULL.
          */
          if (!table && !(table= create_tmp_table(thd, select)))
            DBUG_RETURN(true);
          DBUG_RETURN(table->fill_item_list(&result_columns));
        }
        

        Notice, in the new reduction, the table is created only if select_handler::table is NULL.

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              Assignee:
              bar Alexander Barkov
              Reporter:
              bar Alexander Barkov
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                Updated:
                Resolved: