Auto-increment text primary key in postgres

I would like my content to be searchable by users with a unique 6 characters id (i.e. zab4ac, cba884, ...). I was thinking of creating a id character(6) primary key column, but then I started wondering how to create ids for new rows. They can be random or sequential, it doesn't matter to me, but I was wondering what the best option would be: 1) For every INSERT, do a SELECT to get the last key, and "increment it"? 2) or create a random key and verify if it already exists? Since these seem inefficient (2 queries for every insertion), is there a automated way to do it, like serial, but for characters (000009, 00000a, 00000b, ..., 00000z, 000010, ..., or something more random)? Or should I use serial in the database and convert it to a string for users? I am looking to see if I am missing an easy and efficient approach to the problem.