create agent update attribute shipment based on order release

hi,

i have a DSU like this on Shipment - Created

update shipment set attribute17=(select omv.order_release_gid from order_movement omv where omv.shipment_gid = $gid) where shipment_gid=$gid

i want to update my attribute based on this query.

but what if there is more than 1 order release? let say 1 shipment have 50 order release? 

how much the limit of the text on attribute?

thanks