VACUUM produces odd freespace values

Can anyone explain why VACUUM after INSERT shows steadily decreasing
freespace, while DELETE of the same rows does not decrease consistently?

Specifically, after one row is inserted I see:

SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8128)
(1 row)

but after inserting two more rows and deleting those two rows, I see:

SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8096)
(1 row)

Seems that value should be '(0,8128)'. Is it the unused line pointers
that are causing this?

Another odd thing --- if I change the second VACUUM to VACUUM FULL I
see:

VACUUM FULL mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,0)
(1 row)

There is still a row in the table, so why is there no free space
reported? I realize after VACUUM FULL that only the last page has
freespace --- do we assume that will be used as default for the next
addition and just not bother with the free space map? --- makes sense if
we do that. Does this happen because cluster creates a new relfilenode?

I am attaching the init script, the SQL query script, and the results I
obtained against our CVS HEAD.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +