About index updates
To update an index Cassandra appends data to the commit log, updates the memtable, and updates the index. Writing to a table having an index involves more work than writing to a table without an index, but the update process has been improved in Cassandra 1.2. The need for a synchronization lock to prevent concurrency issues for heavy insert loads has been removed.
When a column is updated, the index is updated. If the old column value was still in the memtable, which typically occurs when updating a small set of rows repeatedly, Cassandra removes the 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.
As with relational databases, keeping indexes up to date is not free, so unnecessary indexes should be avoided.