[GRASS-user] problem with v.db.update and categories

[tommaso]

Hello, I'm trying to polygonize a lines layers. I imported the layer in
grass with v.in.ogr as boundaries, then I attached the centroids with
v.centroid.
Probably because of the polygonize, some attribute are lost in in the
output map, so I have to update the table to fix the attributes:
v.db.update input='polygons' col=building, value='3.0'

After the update, the table look fine with v.db.select: all record have
the right value. But if I export the layer with ogr, some records do not
have attributes at all. My export command looks so.
v.out.ogr format='PostgreSQL' input='polygons' type='area'
dsn='PG:dbname=test_project host=localhost' olayer='test_table'
This warning is raise by v.to.ogr: WARNING: 8 features without attributes  
were written

I guess, the map categories are not synchronized with the database.
v.categories returns 1018 results, while the map contains only 298 records:
v.category polygons option=print | wc
      1018    1018    5133

Where are so many categories coming from? How can I synchronize map and
database?
Thanks, Tomm