Large intermediate results dureing columns update

Given a column defined as bigint (int64) with 1 billion rows. Storage size of this column is 8 GB. Values range in space of int (int32).

UPDATE sapsr3.dfkkop SET vkont_int32 = vkont_int64 ;

This query takes very long (minutes). During execution the data directory grows by 40 GB (!). I would expect the data cells do be converted an written into the new column directly. Even if MonetDB would materialize the whole source column as "intermediate result", I would expect not to exceed 8 GB for "intermediates" and 4 GB for the target column.