The best way to solve the described problem is to create primary key for existing field/fields which uniquely identify each record in the table. However, it could be non-trivial and time consuming task, especially if database is created by third-party vendor or if it has complex structure.

The alternative could be creation of artificial primary keys: i.e., column with automatically filled value, with associated sequence (generator) and trigger. It is necessary to create these objects, fill the values for existing records, create primary key, and then create unique values for this field for every new record. For this purpose we have created the following instruction and templates:

How to create and fill artificial primary keys (often needed for replication) (English version)

Como criar e preencher chaves primárias artificiais (geralmente necessárias para replicação) (Portuguese version)