WL#2184: Allow user to force join method

Affects: Prototype Only   —   Status: Un-Assigned   —   Priority: Medium

This comes #5606 "Nonoptimal join type"

We should generally look at adding new keywords to allow the user to force the 
join method MySQL should use.

To solve this problem, we could use the syntax:

SELECT * from a,b FORCE INDEX ('a') USE JOIN_METHOD range_check ...

This would force MySQL to use the range_check method  if it would be possible.

To fix this particular issue (after the syntax and flag is in place) add a
test to make_join_readinfo() in 'JT_REF:' if we can use the range_check method 
(see in JT_ALL how this can be done)