Consider the log in Figure 19.7. Suppose there is a crash during recovery, justbefore the operation abort log record is written for operation O1. Explain whatwill happen when the system recovers again. If To aborts beforeoperation O, ends, undo ofupdate to C will be physicalBeginning of logTo has completed operation O,on C, releases lower-levellock; physical undo cannot bedone anymore, logical undowill add 100 to CT, can update C since To hasreleased lower-level lock on CTodecidesto abortT, releases lower-level lockon CLogical undo of O, adds 100on CO, undo completeFigure 19.7 Transaction rollback with logical undo operations.

Log-based recovery in database The log is defined as a sequence of log records. A report of…

Answered: If To aborts before operation O, ends,…

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Question

Consider the log in Figure 19.7. Suppose there is a crash during recovery, just
before the operation abort log record is written for operation O1. Explain what
will happen when the system recovers again.

If To aborts before
operation O, ends, undo of
update to C will be physical
Beginning of log
<To start>
<To, B, 2000, 2050>
To has completed operation O,
on C, releases lower-level
<To, O, operatión-begín>
<To, C, 700, 600>
<To, O, operation-end, (C, +100)>
lock; physical undo cannot be
done anymore, logical undo
will add 100 to C
T, can update C since To has
released lower-level lock on C
<T1 start>
To
decides
to abort
<To, O2, operation-begin>
<T,, C, 600, 400>
<T, O2, operation-end, (C, +200)>
T, releases lower-level lock
on C
<To, C, 400, 500>
<To, O, operation-abort>
<To, B, 2000>
<To, abort>
<T, commit>
Logical undo of O, adds 100
on C
O, undo complete
Figure 19.7 Transaction rollback with logical undo operations.

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