pzInsKey is not something that we will define in Keys of a class.pzInsKey will be generated internally by using the Keys defined for a class or class group when the object is saved initially.

Please refer to below link for more info.

https://pdn.pega.com/sites/pdn.pega.com/files/help_v731/procomhelpmain.htm#engine/database/tables/eng-db-pzinskey-property-ref.htm#kanchor4326

I just got little confused, are you referring to "Externally Mapped Class" or OOTB class? If it's an OOTB class, you need to worry about the indexing of the pzInsKey. 

It's a handle, sometimes known as an internal key, is a permanent representation of an object instance key in an external form. A handle uniquely identifies the instance, though in a format that differs from the normal presentation. Internally, the pzInsKey property holds the handle value for the object.

A handle identifies the class, certain key parts, and (for rules and some other classes) the creation date and time of a single instance. Handle values are in the format:

CLASSNAME CLASSKEY ! NAME #YYYYMMDD T HHMMSSTTTTMMM GMT

The primary key column for almost all Pega Platform database tables is named pzInsKey, which corresponds to the pzInsKey property known as the internal key or handle. This permanent key is generated internally by concatenating other properties in the instance and, for rules, from the date and time the object is first saved. The key uniquely defines an instance.

Because the initial portion of the pzInsKey value contains the object class (the value of property pxObjClass), objects of different classes can be stored together in one table with no chance of key collisions.

Refer,

- https://pdn.pega.com/sites/pdn.pega.com/files/help_v72/procomhelpmain.htm#concepts/concepts2/conceptshandle.htm

- https://pdn.pega.com/node/1046676

There will be no performance issue in case of OOTB class.

Hope it answers your query. Kindly notify if it does by marking this post as answered for larger Pega audience.

Regards,

Asif

In my humble opinion, unless there is specific reason why this table shouldn't have pzInsKey as primary key, it should have pzInskey as primary key.

I see 2 reason for this:

 -Performances.

 -Prevent duplication.

PzInskey should be unique. 

If we look a Pega out of the box table, all the one I have checked so far have pzInskey as primary key.

Check for out of the box tables with similar use in your system (like for work cases for instance) wether they do have pzInsKey as primary index. 

Check how the table was created to explain this (manualy, wizard...)

It is better to stick to what Pega software expect to avoid surprise, unless there is a good reason for it.