Join select_handler and Pushdown_select + XPand changes

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.