If you only have a small number of pages that have this problem, then you don't actually need to do anything -- the pages will be marked free by regular vacuuming, and future inserts or updates can make use of those pages. It's not a problem to have a small number of pages in empty state for some time.
So if you're trying to do this, the number of problematic pages must be large.
Not necessarily. I have some tables where I like to use CLUSTER
every 2 or 3 months, to reorganize the data based on an index
and consequently load fewer pages with each call. These tables
don't have more than 2 or 3% of dead records, but they are quite
disorganized from the point of view of that index, since the
inserted and updated records don't follow the order I determined.
