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However, the redo log contains much less information than the
entire data block for the changed data, so it??™s much ???cheaper??? to write to disk. Oracle
writes the redo information to disk as soon as a transaction commits and defers
writing the changed data blocks to the database until several sets of changed blocks
can be written together. Oracle can restore the database using the redo logs, and
these procedures cut down on time-consuming I/O operations.
Figure 8-2. Multiversion read consistency
Alpha
Carol
Darryl
Edward
Frank
Greenie
Beta
Client A
(SCN 112)
111
111
111
111
113
111
111
111
Client B
(SCN 113)
Value SCN
When Client B reads the rows, the changes made
to the "Edward" row with a later SCN are skipped.
198 | Chapter 8: Oracle Multiuser Concurrency
However, when you??™re considering the performance of a database, you have to think
about more than simple I/O operations. It doesn??™t really matter how fast your database
runs if your transaction is waiting for another transaction to release a lock.


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