PostgreSQL functions to get the n-th pseudorandom number generated by random() based on a given seed.

pseudorandom.sql

CREATE FUNCTION pseudorandom(seed DOUBLE PRECISION, n int)
RETURNS DOUBLE PRECISION AS $$ 

    -- Set the seed (optional). When recursing, this is obviously not done.
    SELECT CASE WHEN seed IS NOT NULL THEN setseed(seed) END;
    
    -- recurse until counter reaches 0
    SELECT CASE WHEN n>0 THEN pseudorandom(NULL, n-1) END;
    
    -- run random() once. When counter reached 0, this will be the returned.
    SELECT random();
$$ LANGUAGE SQL;

COMMENT ON FUNCTION pseudorandom IS '
Get 0-based nth pseudorandom number generated by random() with a given seed.
If seed is NULL, then the current seed is used.
';

CREATE FUNCTION pseudorandom_pg(seed DOUBLE PRECISION, n int)
RETURNS DOUBLE PRECISION AS $$ 
BEGIN
    -- Set the seed (optional). When recursing, this is obviously not done.
    IF seed IS NOT NULL THEN
        PERFORM setseed(seed);
    END IF;
    
    FOR i in 0..n-1 LOOP
        PERFORM random();
    END LOOP;
    RETURN random();
END
$$ LANGUAGE plpgsql;

COMMENT ON FUNCTION pseudorandom_pg IS '
Get 0-based nth pseudorandom number generated by random() with a given seed.
If seed is NULL, then the current seed is used.
';