i am preparing for Backup and Recovery paper, my question is that how can be a datafile can contain uncommitted data and after roll forward what will happen to that data.
Sure datafile can contain uncommited data. Imagine a case when you insert 100MB worth of rows in a table in a single transaction, but your redo logs are only 1MB in size. Whenever the log switch ocurs DBWR has to write the whole content of a redo log, either it is commited or not. It can't be any different.
I'm not sure what you mean what will happen to that data during roll forward. Roll forward ocures only during recovery process. If this is what you are interested in, then during roll forvard all that uncomitted data will again be written to the database files. But after roll forward there allways comes *roll back* phase in which all those uncommited data will be rolled back.
ASCII a stupid question, get a stupid ANSI
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