I'm looking to do some reporting on tablespace usage, and wanted to be able to determine how much of the space a table physically occupies is actually in use.
1. I know I can get n_live_tup and n_dead_tup from pg_stat_all_tables, but I don't think that really answers the question entirely. After a vacuum, my understanding is that the n_dead_tup will go to zero, but the physical table size will not have decreased. The pages simply remain available for future use. How do I determine the amount of space which is not occupied by live/dead tuples?
2. Is there a way to calculate the amount of space occupied by dead tuples, or even to calculate the space used by live tuples so I can subtract it from pg_relation_size()? I don't think it's valid to multiply n_dead_tup by page size, as multiple tuples could be on a given page.