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:

CREATE TABLE tbl_Employee
(	
	EmpID INT 
	,EmpName CHARACTER VARYING 
	,CONSTRAINT pk_tbl_Employee_EmpID_EmpName PRIMARY KEY (EmpID,EmpName)
);
 
INSERT INTO tbl_Employee 
VALUES (1,'Anvesh'),(2,'Roy'),(3,'Lee')
,(4,'Nivu'),(5,'Rajesh'),(6,'Nupur');

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.

INSERT INTO tbl_Employee 
VALUES (6,'Noor')
ON CONFLICT (EmpID,EmpName)
DO NOTHING;

Check the result:

SELECT *FROM tbl_Employee;

/*
EmpID   EmpName
-----------------
1       Anvesh
2       Roy
3       Lee
4       Nivu
5       Rajesh
6       Nupur
6       Noor
*/