redbookcover.gif (13955 bytes) Readings in Database Systems, 3rd Edition

Stonebraker & Hellerstein, eds.

Abort, cont.

To perform UNDO, must have a lock on data!

No problem!

Before restoring old value of a page, write a CLR:

You continue logging while you UNDO!!
CLR has one extra field: undonextLSN
Points to the next LSN to undo (i.e. the prevLSN of the record we’re currently undoing).
CLRs never Undone (but they might be Redone when repeating history: guarantees Atomicity!)

At end of UNDO, write an “end” log record.

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1998, Joseph M. Hellerstein.  Last modified 08/17/98.
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