cursors and transactions

Hello,

Please someone could tell me this - Do opening a cursor start a transaction?
What if cursor is select for update?

The problem is that i need to run periodically a pl/sql procedure which opens a cursor for update and searches through a certain table. It MAY or MAY NOT actually update the table row (where current of ...). I cannot integrate the condition into cursor's select statement. When database is inactive (no operations from applications), these procedures do nothing, but if select for update will produce a transaction, it will be archived and there will be spare archived redo logs. So the answer seems to be crucial.

Thanks to anyone, any comments are welcome, Igor

PS.
BTW, Will a single COMMIT or ROLLBACK statement without any preceding DML or something generate redo log entries?

-- 
Igor Sereda,
ITC, Russia