Method init_join_cache_and_keyread

sql/sql_select.cc:34334–34459

@brief Initialize join cache and enable keyread */

    Initialize join cache and enable keyread
*/
void JOIN::init_join_cache_and_keyread()
{
  JOIN_TAB *tab;
  for (tab= first_linear_tab(this, WITH_BUSH_ROOTS, WITHOUT_CONST_TABLES);
       tab;
       tab= next_linear_tab(this, tab, WITH_BUSH_ROOTS))
  {
    TABLE *table= tab->table;
    switch (tab->type) {
    case JT_SYSTEM:
    case JT_CONST:
    case JT_FT:
    case JT_UNKNOWN:
    case JT_MAYBE_REF:
      break;
    case JT_EQ_REF:
    case JT_REF_OR_NULL:
    case JT_REF:
      if (table->covering_keys.is_set(tab->ref.key) && !table->no_keyread)
        table->file->ha_start_keyread(tab->ref.key);
      break;
    case JT_HASH:
    case JT_ALL:
    case JT_RANGE:
      SQL_SELECT *select;
      select= tab->select ? tab->select :
              (tab->filesort ? tab->filesort->select : NULL);
      if (select && select->quick && select->quick->index != MAX_KEY &&
          table->covering_keys.is_set(select->quick->index) &&
          !table->no_keyread)
        table->file->ha_start_keyread(select->quick->index);
      break;
    case JT_HASH_NEXT:
    case JT_NEXT:
      if ((tab->read_first_record == join_read_first ||
           tab->read_first_record == join_read_last) &&
           table->covering_keys.is_set(tab->index) &&
           !table->no_keyread)
        {
          DBUG_ASSERT(!tab->filesort);
          table->file->ha_start_keyread(tab->index);
        }
      break;
    default:
      break;
      /* purecov: end */
    }

    if (table->file->keyread_enabled() &&
        !table->is_clustering_key(table->file->keyread))
    {
      /*
        Here we set the read_set bitmap for all covering keys
        except CLUSTERED indexes, with all the key-parts inside the key.
        This is needed specifically for an index that contains virtual column.

        Example:
          Lets say we have this query