How to generate random unique number in PostgreSQL using function - postgresql

Question

In PostgreSQL, how to generate random unique integer number for column,
return which not exits in table column?

Answer 1

See the pseudo_encrypt function, which implements a permutation based on the Feistel network technique. Combined with a postgres sequence, this guarantees unicity of the result, as well as randomness to the human eye.
Example:
CREATE OR REPLACE FUNCTION pseudo_encrypt(VALUE int) returns int AS $$
DECLARE
l1 int;
l2 int;
r1 int;
r2 int;
i int:=0;
BEGIN
l1:= (VALUE >> 16) & 65535;
r1:= VALUE & 65535;
WHILE i < 3 LOOP
l2 := r1;
r2 := l1 # ((((1366 * r1 + 150889) % 714025) / 714025.0) * 32767)::int;
l1 := l2;
r1 := r2;
i := i + 1;
END LOOP;
RETURN ((r1 << 16) + l1);
END;
$$ LANGUAGE plpgsql strict immutable;
create sequence seq maxvalue 2147483647;
create table tablename(
id int default pseudo_encrypt(nextval('seq')::int),
[other columns]
);
A variant with a 64-bit output space can be found at: pseudo_encrypt() function in plpgsql that takes bigint.
EDIT: pseudo_encrypt implements only one permutation, and it does not accept a user-supplied key. If you prefer having your own permutations, depending on secret keys, you may consider skip32 (a 32-bit block cipher based on Skipjack, with 10 bytes wide keys).
A plpgsql function (ported from Perl/C) is available at:
https://wiki.postgresql.org/wiki/Skip32