Slow query yet indexed – collation mismatch on index?

I had a simple query with 3 tables linked with one inner and one left join. Ran indescribably slowly – unless I changed the left join to an inner join (which I didn’t want to do). Yet all the links were indexed. EXPLAIN showed that the indexes were not being used. Why? Answer – one indexed field had a collation of utf8 and the other of latin1. Aligned that and everything worked nice and fast. Was there any indication from the output of EXPLAIN that this was the issue? No! Anyway, the syntax is DEFAULT CHARSET = latin1 or whatever after the ENGINE = part.