The default
plan is designed to limit the amount of resources used by automated maintenance
tasks such as optimizer statistics gathering, the Automatic Segment Advisor, and the
Automatic SQL Tuning Advisor.
186
Chapter 8 CHAPTER 8
Oracle Multiuser Concurrency 8
Sharing data is at the center of all information systems. As systems provide higher
and higher levels of functionality, we can sometime forget that the ability to efficiently
share data is the underlying governor of overall system performance. At the
same time, database systems must protect the integrity of the data, as the value of
that data is directly proportional to the correctness of the data. Database systems
must protect data integrity, while still providing high levels of performance for
multiuser access. These two forces sometimes conflict and shape some of the core
technology in any database system.
Data integrity must always come first. As Ken Jacobs, vice president at Oracle, put it
in his classic paper entitled ???Transaction Control and Oracle7,??? a multiuser database
must be able to handle concurrently executing transactions in a way that
???ensure(s) predictable and reproducible results.
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