You need Pl/PgSQL for this, as PostgreSQL doesn't support dynamic SQL in its plain SQL dialect.
CREATE OR REPLACE FUNCTION get_cols(target_table text) RETURNS SETOF record AS $$
DECLARE
cols text;
BEGIN
cols := (SELECT STRING_AGG(QUOTE_IDENT(column_name), ', ')
FROM information_schema.columns
WHERE table_name = target_table
AND data_type = 'character varying');
RETURN QUERY EXECUTE 'SELECT '||cols||' FROM '||quote_ident(target_table)||';';
END;
$$
LANGUAGE plpgsql;
However, you'll find this hard to call, as you need to know the result column list to be able to call it. That kind of defeats the point. You'll need to massage the result into a concrete type. I convert to hstore here, but you could return json or an array or whatever, really:
CREATE OR REPLACE FUNCTION get_cols(target_table text) RETURNS SETOF hstore AS $$
DECLARE
cols text;
BEGIN
cols := (SELECT STRING_AGG(QUOTE_IDENT(column_name), ', ')
FROM information_schema.columns
WHERE table_name = target_table
AND data_type = 'character varying');
RETURN QUERY EXECUTE 'SELECT hstore(ROW('||cols||')) FROM '||quote_ident(target_table)||';';
END;
$$
LANGUAGE plpgsql;
Dynamic SQL is a pain, consider doing this at the application level.
