AnsTut10db05 - Page 1 24/12/2009 CP5520 Answer to Tutorial...

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Page 1 24/12/2009 CP5520 Answer to Tutorial Week 10 19.5 What are the before image (BFIM) and after image (AFIM) of data item? What is the difference between in-place updating and shadowing, with respect to their handling of BFIM and AFIM? Answer: The old value of the data item (before updating), is called the before image (BFIM) and the new value (after updating) is called the after image (AFIM). In the in-place updating technique, the updated block is written back to its original disk location. The old values will be overwritten (BFIM will not exists in this scenario). The old value will be available through a copy of the database and/or from the system log. In the shadowing technique, the updated block will be written to a different location (i.e., both the BFIM and AFIM exist in the disk). 19.6 What are UNDO-type and REDO-type log entries? Answer: UNDO-type log entries include the old value (BFIM) of the data item. This is necessary in immediate update and in-place update methods, since the BFIM value is overwritten (in immediate update, there may be an UNDO operation). REDO-type log entries do not include the old value (BFIM) of the data item. For
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This note was uploaded on 12/23/2009 for the course DBST 663 taught by Professor Tba during the Spring '09 term at MD University College.

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AnsTut10db05 - Page 1 24/12/2009 CP5520 Answer to Tutorial...

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