ha_innodb.cc:

  Add a comment that InnoDB always retrieves the row id, if the clustered index was internally generated by InnoDB. Then we know the 'ref' to the row.

sql/ha_innodb.cc

@@ -1983,7 +1983,11 @@ build_template(
 		}
 	    } else if (prebuilt->hint_need_to_fetch_extra_cols
 						== ROW_RETRIEVE_PRIMARY_KEY) {
-		/* We must at least fetch all primary key cols */
+		/* We must at least fetch all primary key cols. Note that if
+		the clustered index was internally generated by InnoDB on the
+		row id (no primary key was defined), then
+		row_search_for_mysql() will always retrieve the row id to a
+		special buffer in the prebuilt struct. */
 
 		fetch_primary_key_cols = TRUE;
 	    }