I think *all* DBD's allow multiple select handles running at the same time on the same database handle. Some DBD's however do not allow two handles on the same table running simultaneously. Other DBD's do not allow mixing simultaneous handles on the same table that alter the table, like select+delete, select+update, update+delete, select+insert, update+insert, insert+delete etc. Most likely it is not the DBD that imposes these restrictions, but the (commit model of the) underlying database.