[Shotwell] relationship thumbnaul/picture in database

[Siegfried Kaiser]

Hello all,
I have a little problem with shotwell - after migrating to a new
computer I miss some pictures, - they are present in the thumbnail list,
but not the image-files themselves. 
I have looked through the database schema, but I find no connection
between thumbnail and image-file.
It would be easy for me to find the image files in my backups, but I
cannot get a list of the missing files because I see no connection in
the database. 
Can anybody explain to me how these data are linke so that I can write
an SQL command which puts out the list of missing images?

Thanks,
             Siegfried