Postgres 12

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CREATE TABLE tbl(id serial PRIMARY KEY, col1 int, col2 int, col3 int, other1 int);
INSERT INTO tbl(col1, col2, col3, other1)  VALUES
  (1,2,3,8)
, (4,5,6,8);

CREATE TABLE

INSERT 0 2

-- Step 1: generate statement
SELECT 'SELECT concat_ws(''**'', '
     || string_agg(quote_ident(attname), ', ') FILTER (WHERE attname LIKE 'col%')
     || ') AS concat_col, '
     || string_agg(quote_ident(attname), ', ') FILTER (WHERE attname NOT LIKE 'col%')
     || ' FROM tbl;'
FROM   pg_attribute
WHERE  attrelid = 'tbl'::regclass
AND    attnum > 0
AND    NOT attisdropped;

?column?

SELECT concat_ws('**', col1, col2, col3) AS concat_col, id, other1 FROM tbl;

SELECT 1

-- Step 2: execute
SELECT concat_ws('**', col1, col2, col3) AS concat_col, id, other1 FROM tbl;

concat_col
id
other1

1**2**3
1
8

4**5**6
2
8

SELECT 2