When you `DROP` / recreate an SAI index, you are not blocked from entering queries that do not use the index.
However, you cannot use that SAI index (based on the same column) until it has finished building and is queryable.
To determine the current state of a given index, query the `system_views.sai_column_indexes` virtual table.
Example:

[source,language-cql]
----
SELECT is_queryable,is_building FROM system_views.sai_column_indexes WHERE keyspace_name='keyspace'
       AND table_name='table' AND index_name='index';
----
