I am interested to hear feedback on a patch I have submitted for a
feature request, bug #45759 (http://bugs.mysql.com/bug.php?id=45759).
This patch notifies the storage engine of when a full index scan is
about to take place (e.g. "select count(*) from foo use_index(a);").
By default (in handler.h), the storage engine does nothing. Individual
storage engines can choose to do whatever they want in this case to
make full index scans faster.
The purpose of this patch is to give storage engines an opportunity to
be aware of full index scans, and in turn, make optimizations should
they choose to.