set serveroutput on; ): Basically the use of SET SERVEROUTPUT is to display the query answer in SQL *PLUS interface ... Copy the path of your PL/SQL Program and paste the path with (@) sign to run ...

Just type one and press enter, and your statement should run. This is a command to psql, the program that is interacting with our Postgres server, and not a SQL statement. psql commands start with a ...