autovacuum_vacuum_scale_factor

The trigger for a vacuum is autovacuum_vacuum_threshold + autovacuum_vacuum_scale_factor × n_live_tup, and the default factor is 0.2: twenty percent of the table has to die before autovacuum looks at it. On a billion-row table that is 200M dead rows and often weeks between runs. Large tables converge on the opposite recipe: factor 0, and a fixed threshold sized to about an hour of the measured dead-row rate, so vacuum runs on a clock instead of on table size.