Multiversion read consistency
guarantees that a user sees a consistent view of the data she requests. If
another user changes the underlying data during the query execution, Oracle maintains
a version of the data as it existed at the time the query began. If there were transactions
underway but uncommitted at the time the query began, Oracle will ensure that
the query ignores the changes made by those transactions. The data returned to the
query will reflect all committed transactions at the time the query started.
This feature has two dramatic effects on the way queries impact the database. First,
Oracle doesn??™t place any locks on data for read operations. This means that a read
operation will never block a write operation. Even where the database places a single
lock on a single row as part of a read operation, a single lock can still cause contention
in the database, especially since most database tables tend to concentrate update
operations around a few ???hot spots??? of active data.
Oracle??™s Isolation Levels | 191
Second, a user gets a complete ???snapshot??? view of the data, accurate at the point in
time that the query began.
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