Lock contentions arise when one transaction attempts to read or write a piece of data, a field, row, table, or schema, that has been locked, from reading, writing, or both by another transaction process.
Lock contentions can often cause long return times, and are best diagnosed with the use of log files.
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.