[GRASSLIST:4154] reading coordinate values from shapefile

[Daniel Isenegger]

Hi list,
thanks to Radim's hint, i am able now to maintain attribute tables on 
postgres.
I'm not sure if it is a stupd question, but the next problem i didn't 
found a solution in the tutorials or FAQs is:
how to extract coordinate values from certain elements of a shapefile 
(for example the x/y-coordinates of a centriod of a certain polygon), 
but not interactively, but with specifying the element?
for example smth in this way:
select X_COOR, Y_COOR from shapefileXY where object_id = 1

Or is it better using ascii_files?  but how to relate there coordinate 
values with elements of the file specified in the attribute tables?

Thanks dani