>I think it's related to the problem of figuring out how many dead tuples you expect to find in the overall heap, which you need to do to have >any hope of this being a comprehensive estimate.
An estimate of number of index scans while vacuuming can be done using estimate of total dead tuples in the relation and maintenance work mem.
n_dead_tuples in pg_stat_all_tables can be used as an estimate of dead tuples.
Following can be a way to estimate,
if nindexes == 0
index_scans =0
else if pages_all_visible
index_scans =0
else
index_scans = Max((n_dead_tuples * space occupied by single dead tuple)/m_w_m,1)
This estimates index_scans = 1 if n_dead_tuples = 0 assuming lazy scan heap is likely to find some dead_tuples.
If n_dead_tuples is non zero the above estimate gives a lower bound on number of index scans possible.
