> How " select count(*) from table_where_you_delete_the_rows "
> clean the garbage ?

This commnd will internally fetch all rows; for each row it will
- look at its transaction_id (to decide whether your transaction can see
that row or not, and hence count it)
-look whether there are older versions of that row,
old enough that they can be garbage collected;
if yes, then (depending on the version of server)
- the old version is removed (garbage collected)
or
- the garbage collection thread is notified that this database page
contains garbage

Ivan