Question on full vacuum clearing waste space

[Wenjun Che <wenjun@xxxxxxxxxx>]

Hi

I am testing full vacuum with pg 10.10 on AWS RDS.  I noticed for some tables, the number of waste bytes stays at a few MB after I run full vacuum.  I double-checked that there are no long running transactions, no orphaned prepared transactions and no abandoned replication slots. 

Here is output from full vacuum for one of the tables:

VACUUM(FULL, ANALYZE, VERBOSE) app_events_users
vacuuming "app_events_users"
"app_events_users": found 0 removable, 1198881 nonremovable row versions in 13369 pages
analyzing "licensing.app_events_users"
"app_events_users": scanned 13369 of 13369 pages, containing 1198881 live rows and 0 dead rows; 30000 rows in sample, 1198881 estimated total rows

What else can prevent full vacuum from reclaiming all waste space ?

Thank you