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Oracle uses rollback segments to
reproduce data as it existed at the time the query started. This capability avoids
the unpleasant choice between allowing queries to see uncommitted data
(known as dirty reads) or having readers block writers (and vice versa). It also
provides a consistent snapshot view of data at a single point in time.
Shared SQL
The parsing of a SQL statement is fairly CPU-intensive. Oracle caches parsed
and optimized SQL statements in the shared SQL area within the shared pool. If
another user executes a SQL statement that is cached, the parse and optimize
overhead is avoided. The statements must be identical to be reused; no extra
spaces, line feeds, or differences in capitalization are allowed. OLTP systems
involve a large number of users executing the same application code. These systems
provide an ideal opportunity for reusing shared SQL statements.
212 | Chapter 9: Oracle and Transaction Processing
Stored outlines
Oracle8i added support of execution-plan stability, sometimes referred to as
bound plans, with stored outlines.


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