pgstattuple, vacuum and free_space

Hi:

  I did the following after installing the pgstattuple contrib code:

 select * from pgstattuple('wind');
-[ RECORD 1 ]------+----------
table_len          | 224854016
tuple_count        | 1492601
tuple_len          | 207535124
tuple_percent      | 92.3
dead_tuple_count   | 11569
dead_tuple_len     | 1361848
dead_tuple_percent | 0.61
free_space         | 3311416
free_percent       | 1.47


vacuum verbose wind;
INFO:  vacuuming "public.wind"
INFO:  index "wind_pkey" now contains 1492601 row versions in 4152 pages
DETAIL:  11569 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.56s/1.77u sec elapsed 36.73 sec.
INFO:  index "wind_measurement_date_index" now contains 1492601 row
versions in 4156 pages
DETAIL:  11569 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.43s/1.83u sec elapsed 35.90 sec.
INFO:  "wind": removed 11569 row versions in 371 pages
DETAIL:  CPU 0.03s/0.03u sec elapsed 2.38 sec.
INFO:  "wind": found 11569 removable, 1492601 nonremovable row versions
in 27448 pages
DETAIL:  0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 2.01s/3.95u sec elapsed 79.39 sec.
VACUUM

select * from pgstattuple('wind');
-[ RECORD 1 ]------+----------
table_len          | 224854016
tuple_count        | 1492601
tuple_len          | 207535124
tuple_percent      | 92.3
dead_tuple_count   | 0
dead_tuple_len     | 0
dead_tuple_percent | 0
free_space         | 4673256
free_percent       | 2.08


My question: when you vacuum a table and generate 'free_space', who is
allowed to consume this 'free_space'? Is it released to the OS for
general use? Or is it reserved just for the database? If the latter, is
it reserved just for 'wind' (in this case)?

Thanks again!