General Concurrency and Performance
As discussed in Chapter 8, Oracle has excellent support for concurrency and performance
in OLTP systems. Some of the key features relevant to OLTP are as follows:
Nonescalating row-level locking
Oracle locks only the rows a transaction works on and never escalates these
locks to page-level or table-level locks. In some databases, which escalate row
locks to page locks when enough rows have been locked on a page, contention
can result from false lock contention when users want to work on unlocked rows
but contend for locks that have escalated to higher granularity levels.
Multiversion read consistency
Oracle provides statement-level and transaction-level data consistency without
requiring read locks. A query is guaranteed to see only the data that was
committed at the time the query started. The changes made by transactions that
were in-flight but uncommitted at the time the query started won??™t be visible.
Transactions that began after the query started and were committed before the
query finishes also won??™t be seen by the query.
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