* Jeff Janes:
> I don't see the virtue of this in this case. Since the index is not
> unique, why not just put the index on (a,b,c,d) and be done with it?
AFAICT, SQLite 4 encodes keys in a way that is not easily reversed
(although the encoding is injective, so it's reversible in principle).
Therefore, covered columns need to be listed separately, so they are
not subject to the key encoding.
