Oracle offers different routes for its database bug fix and security patches such as CPU (Critical Patch Updates) and PSU (Patch Set Updates). CPU is now referred as SPU (security patch update). Among these types of patches, there are similarities and differences, and one may include the other, that’s very confusing. This article discusses key features of these patch routes in order to distinguish their applications.
CPU Patches only contain security related fixes. They are collection of high-priority fixes for security related issues. CPUs are cumulative with respect to prior security fixes but may contain other fixes in order to address patch conflicts with non-security patches.
CPU patches are only applicable to the Database Home. Oracle Support mentioned that "while there is no harm in applying the CPU patch to your CRS home, it is a completely unnecessary step that should not be performed" as stated in Oracle Support document entitled: RAC: CPUApr2009 Should Not Be Applied to CRS home [ID 807790.1].
Oracle Support actually recommends PSUs over CPUs in all cases. This is because PSUs are more comprehensive than CPUs, and PSU patches undergo more rigorous testing than do CPU patches.
Applying a PSU patch will increase the version number. For example, a PSU may change 11.2.0.2.0 on the fifth digit to a new version 11.2.0.2.x, but we don't necessarily consider this as a full-blown version increase since we're only providing fixes to functionality that already exists in your 4-digit version - we are not providing new functionality.
For PSU upgrade, it is required to upgrade Grid Infrastructure before upgrading the database.
Instead of patching PSU to upgrade database to higher version, such as from 11.2.0.2 to 11.2.0.3, we can select to update only to the highest version of 11.2.0.2, such as from 11.2.0.2.0 to 11.2.0.2.10. But patching to the latest PSU for 11.2.0.2 is a completely different 'strategy' than upgrading to 11.2.0.3 - since 11.2.0.3 does offer new functionality whereas any patch for 11.2.0.2 is only going to offer fixes for the functionality that exists.
Some of the PSU patches for lower version like 11.2.0.2 are required to be applied as prerequisites / preparation for upgrading to higher version as 11.2.0.3. So a 2-phased approach (even if you only plan to implement the first phase (patching to latest 11.2.0.2.X patch) is very highly recommendable.
Both PSUs and CPUs are released quarterly on the same cycle.
Both PSUs and CPUs are cumulative. Thus we need only apply the latest one.
PSUs and CPUs are NOT compatible meaning if you apply the 11.2.0.2.2 Database PSU and then want to apply the 11.2.0.2 July CPU this would result in the rollback of the 11.2.0.2.2 Database PSU. That said, once a PSU patching strategy is adopted it must be maintained.
If you apply a CPU now, you can later adopt a PSU patching strategy. (Historically speaking we had CPUs before we ever even offered PSUs so this had to be made possible ...)
The same utility tool, opatch, should be used for both PSU and CPU. Oracle Support reconds downloading the latest available opatch version at the time of upgrade.
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