Update: more testing has shown this to not work as well as I thought...details will follow
Update 2: this doesn't work at all. I need to give it some more thought...M
I've made no secret that 'advisory locks' is one one of the coolest features in PostgreSQL (see my posts here and here). As a general purpose database mutex, you can do all sorts of wonderful things with them...long term locks, cooperative locks, sub transaction locks, etc.
But what about sub statement locks?
Consider the gapless sequence as described here (normal sequences produce 'holes' in the number line when rollbacks or database crashes occur). In the example, an external record is used to track the counter and provide concurrency protection. Well, how about this:
create table t(id int primary key);
insert into t select coalesce(id, 1) from
id + 1 as id,
pg_advisory_unlock(1) from t
order by id desc limit 1
The above statement (ab)uses the left to right order of execution of the fields of the statement. No external object is locked, and the outer select statement could do a bunch of other time consuming things without tying up the lock and killing concurrency. There is no chance of two transactions grabbing the same value of id in read committed mode (there may be a small chance of a race to grab the first value of id). This may not be the best or even a good way to do this, but it's an interesting demonstration of the technique.