Instagram's sharding function. (see http://instagram-engineering.tumblr.com/post/10853187575/sharding-ids-at-instagram for explanation)

instragram_next_id.sql

CREATE OR REPLACE FUNCTION insta5.next_id(OUT result bigint) AS $$
DECLARE
    our_epoch bigint := 1314220021721;
    seq_id bigint;
    now_millis bigint;
    shard_id int := 5;
BEGIN
    SELECT nextval('insta5.table_id_seq') %% 1024 INTO seq_id;

    SELECT FLOOR(EXTRACT(EPOCH FROM clock_timestamp()) * 1000) INTO now_millis;
    result := (now_millis - our_epoch) << 23;
    result := result | (shard_id << 10);
    result := result | (seq_id);
END;
$$ LANGUAGE PLPGSQL;

CREATE TABLE insta5.our_table (
    "id" bigint NOT NULL DEFAULT insta5.next_id(),
    -- ...rest of table schema...
)