Dealing with disk space full in postgresql
Because PostgreSQL must write WAL before making any changes to tables, it needs free disk space in order to delete things and release more disk space.

If you let the disk fill up, you can't recover from within PostgreSQL. Even TRUNCATE still has to write to WAL.

So you must free some space on the volume, or expand the volume. If your PostgreSQL log files are in pg_log in the data directory, you can safely remove some of those and restart Pg.

Do not delete pg_xlog or pg_clog. These are not server error logs, they're critical parts of the database, the transaction log and commit log.