Fix problem where db.set_author forced a recalculation of author_sort whenever editing metadata of a book with a unique author (only book with that author). The problem arose because the author was deleted from the DB then recreated, causing the trigger to be fired.

src/calibre/gui2/dialogs/metadata_single.py

@@ -760,6 +760,7 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
                         _('Could not open %s. Is it being used by another'
                         ' program?')%fname, show=True)
             raise
+        self.db.clean()
         self.save_state()
         QDialog.accept(self)
         if callable(self.accepted_callback):

src/calibre/library/database2.py

@@ -1126,7 +1126,6 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
         if not authors:
             authors = [_('Unknown')]
         self.conn.execute('DELETE FROM books_authors_link WHERE book=?',(id,))
-        self.conn.execute('DELETE FROM authors WHERE (SELECT COUNT(id) FROM books_authors_link WHERE author=authors.id) < 1')
         for a in authors:
             if not a:
                 continue