CREATE TABLE a (id int, fname text, lname text, email text, phone text);
INSERT INTO a VALUES
(1, 'john', 'doe', 'john@gmail.com', null),
(2, 'peter', 'green', 'peter@gmail.com', null);

CREATE TABLE b (id int, fname text, lname text, email text, phone text);
INSERT INTO b VALUES
(null, 'peter', 'glover', 'bob@gmail.com', '777'),
(null, null, 'green', 'peter@gmail.com', '666');

fname = 0.1
lname = 0.3
email = 0.5
phone = 0.5

(2, 'peter', 'green', 'peter@gmail.com', null) and
(null, null, 'green', 'peter@gmail.com', '666') is 0.8 (lname + email)

(2, 'peter', 'green', 'peter@gmail.com', null) and
(null, 'peter', 'glover', 'bob@gmail.com', '777') is 0.1 (fname)

id, fname, lname, email, phone, similarity
2,  null,'green', 'peter@gmail.com', '666', 0.8

create or replace function a_b_similarity(a, b)
returns numeric language sql as $$
    select
        coalesce(($1.fname = $2.fname)::int, 0) * 0.1 +
        coalesce(($1.lname = $2.lname)::int, 0) * 0.3 +
        coalesce(($1.email = $2.email)::int, 0) * 0.5 +
        coalesce(($1.phone = $2.phone)::int, 0) * 0.5
$$;

select *, a_b_similarity(a, b)
from b
cross join a

 id | fname | lname  |      email      | phone | id | fname | lname |      email      | phone | a_b_similarity
----+-------+--------+-----------------+-------+----+-------+-------+-----------------+-------+----------------
    | peter | glover | bob@gmail.com   | 777   |  1 | john  | doe   | john@gmail.com  |       |            0.0
    | peter | glover | bob@gmail.com   | 777   |  2 | peter | green | peter@gmail.com |       |            0.1
    |       | green  | peter@gmail.com | 666   |  1 | john  | doe   | john@gmail.com  |       |            0.0
    |       | green  | peter@gmail.com | 666   |  2 | peter | green | peter@gmail.com |       |            0.8
(4 rows)