avoid useless group.derive() in more cases, i.e., not only in case the tail of the map-BAT is (marked as) key (unique), but also in case the extend-BAT already contains as many groups as there are tuples (conservatively handle the case that map-BAT and value-BAT are not of same length;  although it is not clear whether that case can occur at all ...)
avoid useless group.derive() in more cases,
i.e.,
not only in case the tail of the map-BAT is (marked as) key (unique),
but also in case the extend-BAT already contains as many groups as there are tuples
(conservatively handle the case that map-BAT and value-BAT are not of same length;
 although it is not clear whether that case can occur at all ...)

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb