a fix (bug #10303: Misleading Last_query_cost value).

sql/sql_select.cc:
  a fix (bug #10303: Misleading Last_query_cost value).
  reset last_query_cost in the very beginning.

sql/sql_select.cc

@@ -541,6 +541,9 @@ JOIN::optimize()
     DBUG_RETURN(0);
   optimized= 1;
 
+  if (thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS)
+    thd->status_var.last_query_cost= 0.0;
+
   row_limit= ((select_distinct || order || group_list) ? HA_POS_ERROR :
 	      unit->select_limit_cnt);
   /* select_limit is used to decide if we are likely to scan the whole table */