sql select (column-name) as subquery in postgreSQL [duplicate]

I am trying to have a SQL statement where the column names in the SELECT are a subquery. The basic format is:

SELECT (<subquery for columns>) FROM Table;

My subquery returns 4 rows of field names, so I need to make them a single row. I used:

SELECT array_to_string(array_agg(column_names::text),',') FROM Fieldnames;

And then I get a returned format of col1, col2, col3, col4 for my 4 returned rows as a string. If I paste in the raw test for my query, it works fine as:

SELECT (col1, col2, col3, col4) FROM Table;

The issue arrises when I put the two together. I get an odd response from psql. I get a:

?column?
col1, col2, col3, col4

with no rows returned for:

SELECT(SELECT array_to_string(array_agg(column_names::text),',') FROM Fieldnames) FROM Table;

Conceptually, I think there are two ways I can address this. I need to get my subquery SELECT back in a format that I can put as the column-name argument to the first SELECT statement, but because I return multiple rows (of a single value of a varchar for the column name that I want), I thought I could just paste them together but I cannot. I am using psql so I do not have the "@" list trick.

Any advice would be appreciated.

Solution: Here is why the question is not a duplicate, and how I solved it. In trying to simplify the question to be manageable, it lost its muster. I ended up writing a function because I couldn't use @ to pass a list to SELECT in postgreSQL. When you want to select only a subset of rows, you cannot pass a nested (SELECT) even with an AS, although this works in Oracle. As a result, I wrote a function that effective created a string, and then passed it as the SELECT. There seems to be something fundamentally different on how the SQL parser in PostgreSQL handles the arguments for SELECT from Oracle, but everyone DB is different.

Thanks!