Originally posted by DaPi
I think the problem is low cardinality, which tends to mean that a condition on that column is not very selective. If (to take an extreme case) 1% have col1='XXX' and 99% have col1='YYY' (which is a bit odd if it is CHAR(2)!) then the select for 'XXX' works best by index, 'YYY' by full table scan (probably).
(I have no answer for Q1)
Unfortunatly the low cardinality is aparently not the problem ...
The predicate select about 5% of the table's rows.