Including set type in index

[Angus]

I'm trying to index a set properly, but I can't seem to get it to work with my queries. If I use an EXPLAIN like this

EXPLAIN SELECT * FROM t1 WHERE (set_col IS NULL OR FIND_IN_SET('set_datum',set_col)=0) AND a_varchar IN ('string1','string2',...,'stringn') AND (bool1=1 AND bool2=0)

and I have a unique index of set_col,a_varchar,bool1,bool2, it lists this index as a possibility, but then goes and uses another unique index that holds just a_varchar. How do I get it to use a more complete index?

Something else unexpected: the "key_len" from the EXPLAIN gives 52, but if I call

SELECT LENGTH(a_varchar) FROM t1 ORDER BY LENGTH(a_varchar) DESC

the first row returned is only 27.