Covering Indexes

Hackers,

Very interesting design document for SQLite 4:
 http://www.sqlite.org/src4/doc/trunk/www/design.wiki

I'm particularly intrigued by "covering indexes". For example:
   CREATE INDEX cover1 ON table1(a,b) COVERING(c,d);

This allows the following query to do an index-only scan:
   SELECT c, d FROM table1 WHERE a=? AND b=?;

Now that we have index-only scans in 9.2, I'm wondering if it would make sense to add covering index support, too, where additional, unindexed columns are stored alongside indexed columns.

And I wonder if it would work well with expressions, too?

David