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Basics of Instance Recovery and Database Crash recovery

Oracle introduced deferred rollback segment from 7.3 version and thus made DB startup time quicker in case Oracle had to recover the instance/DB during startup.  More later. It wont be small post.

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Your database has massive dmls being executed and suddenly power failure occurs. Now when you start DB after outage your DB may take long time to open if it is oracle version 7.3 , as oracle has first to roll forward redo logs and then roll back uncommited changes before opening the database. From oracle 7.3 since oracle has deferred rollback segments DB is opened immediately after roll forward phase occurs and rollback phase of instance recovery takes place in background while DB is opened and is accessible.

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