GNUe-Forms: computing totals

Hello everyone,

I'm trying to build a form that contains a block (grid-mode) 
and additionally a field that contains the total corresponding 
to one of the block's fields. 
The total_field would be recalculated after committing (inside
a POST-COMMIT trigger). 

I have read the chapter on datasources in the "gnue-common 
developer's guide", but it is unclear to me whether i should use
the DataSourceWrapper class is what i need or not.  

In fact, my question is :  
   How do I execute (inside a gnue-forms trigger) an arbitrary SQL
   command like "select sum(coalesce(SOME_COLUMN,0)) from SOME_TABLE"
   and how do i gather the returning values ?


Thanks in advance for any tip!

Adrian Maier