Problem:
Loose index scan is not chosen if a query has a equality condition.

Analysis:
Optimizer indeed chooses loose index scan as the cheapest possible access
plan for the given query before it checks for ref access.
However optimizer chooses ref access without calculating the cost because
of the heuristic "ref scan is always cheaper compared to range scan if it
is utilizing the same or more keyparts ". However in case of loose index
scan, it is not always guranteed that ref scan will be looking at
same/less number of qualifying rows.

Solution:
Do not consider the above heuristic when the chosen access plan is loose
index scan. Instead calculated the cost for doing ref access before choosing
it.