About index updates
As with relational databases, keeping indexes up to date is not free, so unnecessary indexes should be avoided. When a column is updated, the index is updated as well. If the old column value was still in the memtable, which typically occurs when updating a small set of rows repeatedly, Cassandra removes the corresponding obsolete index entry; otherwise, the old entry remains to be purged by compaction. If a read sees a stale index entry before compaction purges it, the reader thread invalidates it.