Error in passing columns as parameters in procedures in postgre using ems sql manager

Question

I have 2 tables order table and customer table. The order table contains order_id, customer_id where order_id are the different orders placed by a customer and customer_id is the customer id(general). The customer table contains customer_name and customer_id. The relationship between order table and customer table is customer_id column.

I was trying to use procedure in PostgreSQL using ems sql manager. The procedure will take column name as a parameter and will return the values in the column from a table by using the below procedure. Here we are trying to find values in customer_name column from customer table

create or replace function customers(name TEXT)
returns table(cust_name TEXT)
as
$$
    SELECT $1
    from customer
$$
language sql;

select customers('customer_name');

Few comments here - 1. Though I have specified that I am using postgresql I have specified sql in the language because language plpgsql was not working. 2. This procedure returns 1 column repeating customer_name as value for all the records present in customer table. I guess it is taking customer_name as text value and not using it as column name in the select system.

To correct the 2nd comment, I used this link in stack overflow which uses execute statement in passing columns as parameters - Define table and column names as arguments in a plpgsql function?

So, I copied the 2nd example and tried implementing that also -

CREATE OR REPLACE FUNCTION customers(name regclass)
  RETURNS table(cust_name TEXT)AS
$func$    
   EXECUTE 'SELECT '|| name ||' FROM CUSTOMER';
$func$ LANGUAGE sql;

But this is throwing an error syntax error at execute statement.

So, I tried another way which too was given in stack overflow - Refactor a PL/pgSQL function to return the output of various SELECT queries

The code is here -

CREATE OR REPLACE FUNCTION customers(name regclass)
      RETURNS table(cust_name TEXT)AS
    $func$    
       EXECUTE 'SELECT name FROM CUSTOMER' using name;
    $func$ LANGUAGE sql;

select customers('customer_name');

But this is also throwing syntax error at execute.

After exhausting all the options, I am confused how to pass column names in the procedure. Please note that Declare statement is also not working which is also given in one of the posts in stack overflow.

Can somebody provide me a correct query to pass columns as a parameter in procedures/stored functions? Thanks in advance.

Please note that I am using Postgre Sql version 8.4 and EMS Sql Manager 5.6 version.