Chapter 19, Problem 9PE

Explanation of Solution

a.

Problem occurred when rolling back first transaction:

• Consider the given case where another transaction takes up the space generated by the deletion of a record by an uncommitted transaction.
• In case, if the first transaction needs to be rolled back, then
  • The record deleted by the first transaction is restored.
  • When the usual physical manner undo is performed with the old record values;
    • The space that was reallocated to the new record by the second transaction would get overwritten by this undo...

Explanation of Solution

b.

Issues:

• When page-level locking is used, this problem will not crop up because of the following reasons:
  • The free space gener...

Explanation of Solution

c.

Solving the problem:

In order to resolve the problem using tuple-locking the following methods are followed:

• Defer the freeing up of space until a transaction commit.
• Here, ensure that the space will be freed even when a system fails just after the commit.
• For this, the log records are modified for including the information about the space being freed, which should have been performed after the commit.
• In fact, all such operations can be performed as a transaction along generating their log records, followed by the post-commit log records...