Just to elaborate further. 

Every Pega Platform internal database table in data or rulese schema contains a pzInskey column and is the primary key to the table. When we open a record from the database to get full data in the blob , rule or data instance, we use the pzInsKey as it's unique for the table .

Example: select pzpvstream from <pega_table> where pzInskey = '<key_value>'

The key(s) to a table, as defined in a class,  can be made up one or more exposed properties to the table. The values become part of the pzInskey.

Take for example the Data-Admin-Operator-ID table.

Table: <pr_data_schema>.pr_operators

Primary Key at database level: pzInkey (Unique = yes)

Class Key (as defined in Class record):  pyUserIdentifier

Example colum pzInskey:  "DATA-ADMIN-OPERATOR-ID KOYLC@PEGA.COM"

Example column pyUserIdentifier: "koylc@pega.com"

Methods Obj-Open vs Obj-Open-By-Handle:

These two activity methods can be used to open an instance.  At run time they both generate the same SQL. 

When using Obj-Open you specify a class and then the property or set of properties that make up the keys as defined in class rule. 

So for Obj-Open on Data-Admin-Operator-ID I would use a propertyName of .pyUserIdentifier and then a value. 

Example: 

At run time and from tracer with DB query enabled you can see the SQL:

The Obj-Open method is building the pzInsKey value for you. 

The Obj-Open-By-Handle method allows you to directly provide the pzInsKey value.

Example:

The InstanceHandle in this case is all upper case: "DATA-ADMIN-OPERATOR-ID KOYLC@PEGA.COM".

The Step page has to be of the same class. 

At runtime it's the same SQL. 

To answer your question:

You are already using the primary key of the table, pzInkey, when using an Obj-Open step. 

Hope this answers your question.