Many posts/questions on external classes and ppl have different picture of what an external class's definition is, in Pega. I think external class concept is made simple now in Pega starting 7.2. I'm not sure though. someone can confirm.

External class - Pega 7.1.9 help URL - Talks about lot of stuff on external class.

External class - Pega 7.2 help URL -- Help page here is a cleaned one. It doesn't talk about pzInsKey, nothing about can't create declare index etc. Probably now, external class is just simple. It just maps to a table in another database, but has all features of a internal class.

In latest version, I also noticed that when I add a record to external class, Pega is adding a "pzInsKey" property to that record. May be they do this to , to be able to efficiently open records using "Obj-Open-By-Handle" on external records (that have multiple columns as primary key). But wait, since the external table doesn't have pzInsKey column, therefore no index on that column, and therefore can't be an efficient obj-open-by-handle. Then why at all does Pega add pzInsKey to the external class's instance record?