Update: On closer inspection, the lock type was not on the table, but on a tuple. For more information on this locking mechanism see the internal Postgresql tuple locking documentation. Postgres does not have lock promotion as suggested in the debugging section of this post.
I maintain an internal-facing service at Heroku that does metadata processing. It's not real-time, so there's plenty of slack for when things go wrong. Recently I discovered that the system was getting bogged down to the point where no jobs were being executed at all. After hours of debugging, I found the problem was an UPDATE
on a single row on a single table was causing the entire table to lock, which...