The values of artefact,usr columns of the table are not necessarily
unique.  Perhaps a better option would be to create an id column.
This primary key was added in a3f6fbfe

.

Change-Id: Ib151a14f0943d799d45a7697abc00f117883cd95
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>