sometimes use hibernate i see following warning:
[code]17.12.2003 14:54:51 net.sf.hibernate.impl.SessionImpl finalize
WARNING: afterTransactionCompletion() was never called[/code]
what i can do with it?

I believe this error occurs when you don't call transaction.commit();

I would appreciate a more seasoned hibernate veteran explaining the implications of such a warning message.

For instance:

is it necessary to call transaction.commit() after simply retreiving some rows from the database through a select statement?

You should allways do everything database related inside a transaction, this includes simple read operations.