blah.sql

asdf=# create table foo(id int, version int);
CREATE TABLE
Time: 2.335 ms

asdf=# create function version_err() returns trigger as $$
 begin
  raise exception 'wrong version';
end;
$$ language plpgsql;
CREATE FUNCTION
Time: 5.548 ms

asdf=# create trigger version_check_trigger 
    before update on foo
    for each row
    when (NEW.version != OLD.version + 1)
    execute procedure version_err();
CREATE TRIGGER
Time: 7.160 ms

asdf=# insert into foo(id, version) values (1, 1);
INSERT 0 1
Time: 2.993 ms

asdf=# insert into foo(id, version) values (2, 1);
INSERT 0 1
Time: 0.396 ms

asdf=# update foo set version=2 where id=1;
UPDATE 1
Time: 3.465 ms

asdf=# update foo set version=2 where id=1;
ERROR:  wrong version
Time: 0.695 ms

asdf=# update foo set version=2 where id=1;
ERROR:  wrong version
Time: 0.550 ms

asdf=# update foo set version=3 where id=2;
ERROR:  wrong version
Time: 0.533 ms