VACUUM does a good job and it will reclaim free space as needed. However, when can VACUUM actually clean out rows and turn them into free space? The rule is this: if a row cannot be seen by anybody anymore, it can be reclaimed. In reality, this means that everything that is no longer seen even by the oldest active transaction can be considered to be really dead.
This also implies that really long transactions can postpone cleanup for quite some time. The logical consequence is table bloat. Tables will grow beyond proportion and performance will tend to go downhill. Fortunately, starting with PostgreSQL 9.6, the database has a nice feature that allows the administrator to intelligently limit the duration of a ...
