PostgreSQL 9.5: Multiple columns or keys in ON CONFLICT clause

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I already a shared article on ON CONFLICT clause of PostgreSQL 9.5

PostgreSQL 9.5: Insert IF not Exists, Update IF Exists (Insert ON CONFLICT option)

In above article, I used only one single key in ON CONFLICT clause.

A day before yesterday, I got an email like, does it require to add a unique index on those columns which we require to be in ON CONFLICT clause?
The answer is: Yes

You must need to define a unique index on those columns which you are planning to use in ON CONFLICT clause because it can only check the duplicates bases on unique indexes only.

Below is a demonstration of this:

Create a table with sample data with composite PRIMARY KEY:

Try to insert a duplicate EmpID record, using option INSERT ON DO NOTHING:
It will insert a new record because key EmpID + EmpName is not duplicate. But if you execute the same, insert the second time, it will do nothing.

Check the result: