- The basic rule of read consistency is that the query must return the
data as it existed at the time the query started. Changes that were
committed after the query began may be inconsistent with other data
that the query has already read. So, these changes must be 'rolled
back' for the query and only the query. For example, a company is
running payroll for its employees. At the same time, another person is
updating some salaries to reflect an increase in pay. Without read
consistency, the company would have to hand check the payroll to
determine who was paid the old salary and who was paid the new salary.
For more info on read consistency read the Oracle Concepts manual
(http://download-west.oracle.com/docs/cd/B19306_01/server.102/b14220/intro.htm#sthref134)
- Yes, Tc was committed after QueryA began.
- In terms of the last question, there can be more than 1 transaction
associated with the same data/index block. There can be 1 and only 1
*active* transaction associated with an undo block (but there can be
more than 1 inactive (committed/rolled back) transaction undo
information in an undo block).
Regards,
Daniel Fink
Received on Tue Jan 17 2006 - 08:37:34 CST
