Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

7.2 Oracle and Concurrent User Access

Oracle solves the problems created by concurrent access through a
technology called multiversion read
consistency, sometimes referred to as
MVRC. 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 if a database places a single lock on a
single row as part of a read operation, it can still cause contention
in the database, especially because most database tables tend to
concentrate update operations around a few "hot
spots" of active data.

Second, a user gets a complete
"snapshot" view of the data,
accurate at the point in time that the query began. Other databases
may reduce the amount of contention in the database by locking an
individual row only while it's being read, rather
than over the complete duration of the row's
transaction. A row that's retrieved at the end of a
result set may have been changed since the time the result set
retrieval began. Because rows that will be read later in the
execution of the query weren't locked, they could be
changed by other users, which would result in an inconsistent view of
the data.