Let's create and populate a simple table composed of one partition key a, two clustering keys b & c, and one secondary index on a standard column e:
With such a schema, I am allowed to query on the indexed column without filtering by providing the first two elements of the primary key:
Let's now introduce an index on the first clustering key:
Now, I expect the same query as above to work without filtering, but it's not:
I think this is a bug on the way secondary indexes are accounted for when checking for unfiltered queries.