Starting 11.2, Oracle introduced Standby-First Patch Applywhich provides support between different software releases in the primary and standby. In other words, for certified Oracle patches, it allows the customer to apply/validate the patch first on the standby software while the primary stays in the previous software release. Also it enabled the customer to switch over between primary and standby in the mixed software version.
Patch README should provide the details on whether or not the patch is certified for standby-first patch apply. According to Oracle doc or Oracle Patch Assurance - Data Guard Standby-First Patch Apply (Doc ID 1265700.1), the following patches are certified for standby-first patch apply.
Exadata Database Bundle Patch
Patch Set Update (PSU)
Critical Patch Update (CPU)
Interim (“one-off”) patches
Make sure to keep compatible between primary and standby the same during the patching. It is noted that mixed version cannot be run for more than 31 days (possibly support issue if it goes beyond) and it is only viable for the patches which are not more than 1 year apart. So to be enable to utilize this feature, the following conditions should meet :
Software is 11.2 on-wards
Patch is determined explicitly in readme to be Standby-First Patch apply
Mixed version can only be run less than 31 days
The qualified patch should not be more than 1 year apart from the current primary/standby run.