Algorithms for Recovery and Isolation Exploiting Semantics: Difference between revisions
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=== Undo ===
After the Redo phase, the database reflects the exact state at the crash. However the changes of uncommitted transactions have to be undone to restore the database to a consistent state.
For that we run backwards through the log for each transaction in the TT (those runs can of course be combined into one) using the Previous Sequence Number fields in the records. For each record we undo the changes (using the information in the Undo field) and write a compensation log record to the log file. If we encounter a Begin Transaction record we write an End Log record for that transaction.
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