Problem described in this bug report affects MyISAM tables only.

Running mysqld --flush instructs mysqld to sync all changes to disk
after each SQL statement. It worked well for INSERT and DELETE
statements, but it did sync for UPDATE only in case if there was
index change (change of colum that has an index). If no updated column
has an index, data wasn't synced to disk.

This fix makes UPDATE statement to sync data to disk even if there is
no index change (that is only data change) and mysqld is run with
--flush option.


myisam/mi_update.c:
  Every myisam function that updates myisam table must end with
  call to _mi_writeinfo(). If operation (second param of
  _mi_writeinfo()) is not 0 it sets share->changed to 1, that is
  flags that data has changed. If operation is 0, this function
  equals to no-op in this case.
  
  mi_update() must always pass !0 value as operation, since even if
  there is no index change there could be data change.