Use of index for SELECT DISTINCT

[Dominique PRUNIER]

Hello,

I may be wrong but i find something strange in index use when selecting distinct value. For example, let's assume we have a table like that:

a: integer
b: varchar(128)

Let's say that b has only a few distinct values but the table has MANY rows (2 000 000). The column b is indexed, with of course a VERY low cardinality.

Why optmimizer will always scan the 2 000 000 (using index on b though) to execute query SELECT DISTINCT b FROM table while it knows there are only a few distincts values from the index ?

Thanks