2. Page visibility rate of each table There is no way to know how many page-bits are them of each tables stored in their visibility maps. If we can show this information, then we will be able to guess vacuum overhead for the table. For example, if this table is a very big table but page visibility rate is high, then we can advise pg-users that vacuum for this table will execute faster than they think by low I/O overhead. Furthermore, this information can also be used in order to inform pg-users about "real" index-only-scan usability.
