About Oracle Patches

About Patches and Patch Sets

Oracle issues product fixes for its software called patches. They are associated with particular releases and versions of Oracle products. The patching cycle involves downloading patches, applying patches, and verifying the applied patch to ensure that the bug fixes present in the patch reflect appropriately.
Patching involves migrating from one version of the software product to another, within a particular release, unlike upgrading which involves moving from one release of a product to another newer release of the software product. When you apply the patch to your Oracle software installation, it updates the executable files, libraries, and object files in the software home directory. The patch application can also update configuration files and Oracle-supplied SQL schemas.
Oracle periodically releases the following types of patches to fix the bugs encountered in the Oracle software:

Patch Type	Description
Interim Patches	Released to fix a bug, or a collection of bugs. Previously called patch set exceptions (PSE), one-off patches, or hot fixes.
Interim Patches (for Security bug fixes)	Released to provide customer specific security fixes. Previously referred to as a test patch, fix verification binary, or e-fix.
Diagnostic Patches	Mainly help diagnose and verify a fix, or a collection of bug fixes.
Bundle Patch Updates	Cumulative collection of fixes for a specific product or component. Previously referred to as a maintenance pack, service pack, cumulative patch, update release, or MLR.
Patch Set Updates (PSU)	Cumulative patch bundles that contain well-tested and proven bug fixes for critical issues. PSUs have limited new content, and do not include any changes that require re-certification.
Security Patch Updates	A cumulative collection of security bug fixes. Previously known as Critical Patch Updates, or CPUs.

Interim patches are bug fixes available to customers in response to specific bugs. They require a particular base release or patchset to be installed before you can apply them. These patches are not versioned and are generally available in a future patchset and also the next product release. Interim patches are applied by using Enterprise Manager Cloud Control or OPatch, which is included with your Oracle Database installation.
Patch sets updates (PSUs) and patch bundles are mechanisms for delivering fully tested and integrated product fixes. All the fixes in a patch set have been tested and are certified to work with each other. Because a patch set includes only low impact patches, it does not require you to certify applications or tools against the updated Oracle Database software. When you apply a patch set, many different files and utilities are modified. This results in a release number change for your Oracle software, for example, from Oracle Database 11.2.0.3.0 to Oracle Database 11.2.0.3.2. You use OPatch to apply PSUs.