COPY SELECT * FROM <table> INTO <file>;
DELETE FROM <table>;
COPY INTO <table> FROM <file>;

Which suggest that deleting rows (or cleaning??) table will get back the space.

One was from this question
http://mail.monetdb.org/pipermail/users-list/2011-July/005036.html




1) copy all table contents to another table, drop old table, copy again

I have tried both deleting rows and then droping the table but the result was inconsistant. 
May be monetdb cleans up after some interval

Other Related Questions were
1. http://mail.monetdb.org/pipermail/users-list/2010-March/003952.html


2. FEB 23, 2010

Reeves, Matthew J. wrote:
> Hello,
> 
>    I have a MonetDB database that contains a table of rolling events.
> There are 200 million records with 500,000 bulk loaded each day with the
> oldest 500,000 deleted at midnight. I'm finding the database gets slower
> as time goes on. It gets to the point of being almost unusable after a
> week. Is there something I should be doing different?
Hello Matthew,
Thanks for using MonetDB. Interesting application target.

This is a known issue. The system keeps the deleted tuples and rebuilds the table.
A vacuum-like feature is needed and on the wishlist. For now, taking a copy
of the table would 'solve' your problem.
 regards, Martin
> 
> Thanks,
> Matt
>