Function cost_for_index_read

sql/sql_select.cc:8367–8412

*/

static ALL_READ_COST cost_for_index_read(const THD *thd, const TABLE *table,
                                         uint key, ha_rows records,
                                         bool eq_ref)
{
  ALL_READ_COST cost;
  handler *file= table->file;
  ha_rows max_seeks;
  ha_rows extra_reads= eq_ref ? 0 : 1;
  DBUG_ENTER("cost_for_index_read");

  max_seeks= (ha_rows) thd->variables.max_seeks_for_key;
  set_if_bigger(records, 1);

  if (file->is_clustering_key(key))
  {
    cost.index_cost=
      file->ha_keyread_clustered_time(key, 1, records+extra_reads, 0);
    cost.copy_cost= rows2double(records) * file->ROW_COPY_COST;
    /* There is no 'index_only_read' with a clustered index */
    cost.row_cost= {0,0};
    /* Caping of index_blocks will happen in handler::cost() */
    cost.max_index_blocks= MY_MIN(file->row_blocks(), max_seeks);
    cost.max_row_blocks= 0;
  }
  else if (table->covering_keys.is_set(key) && !table->no_keyread)
  {
    cost.index_cost= file->ha_keyread_time(key, 1, records + extra_reads, 0);
    cost.row_cost= {0,0};
    cost.copy_cost= rows2double(records) * file->KEY_COPY_COST;
    cost.max_index_blocks= MY_MIN(file->index_blocks(key), max_seeks);
    cost.max_row_blocks= 0;
  }
  else
  {
    cost.index_cost= file->ha_keyread_time(key, 1, records + extra_reads, 0);
    /* ha_rnd_pos_time() includes time for copying the row */
    cost.row_cost= file->ha_rnd_pos_time(records);
    cost.max_index_blocks= MY_MIN(file->index_blocks(key), max_seeks);
    cost.max_row_blocks=   MY_MIN(file->row_blocks(), max_seeks);
    cost.copy_cost= 0;
  }
  DBUG_PRINT("statistics", ("index_cost: %.3f  row_cost: %.3f",
                            file->cost(cost.index_cost),
                            file->cost(cost.row_cost)));
  DBUG_RETURN(cost);
}


/**