Try the following alternative with composite partition key to encode the dual states of the boolean:
create table x(
// other fields
primary key ((id,flag)
Your previous "select * from x where flag = true;" translate into:
SELECT * FROM x WHERE id=... AND flag = true
Of course, you'll need to provide the id in any case.
If you want to query only on the boolean flag, I'm afraid that manual indexing or secondary index (beware of cardinality !) are your only choices.