Interesting. Not sure I understand that case. For example in http://sqlfiddle.com/#!2/04659/3  the quotes have no affect - you hit full table scan either way which is totally expected because left join does not restrict number or rows to return so if it returns all rows in left table then it certainly has to scan them all and no benefit comes from using an index anyway. If I add the restraint to WHERE as well then both queries correctly use index regardless of quoting.

If you found the quotes made a difference in this query, could you post the full query or modify my fiddle to show it please? I'm now just interested in understanding this in general :).