Cassandra SAI Indexes: When They Work and When They Don't
· 11 min read
Storage-Attached Indexing (SAI) is Cassandra's modern approach to secondary indexing. It promises efficient queries on non-partition-key columns without the scaling problems of legacy secondary indexes. But SAI has boundaries — and when your query crosses them, Cassandra can end up performing slower than if the index didn't exist at all.
This post walks through how SAI works under the hood, when it can be used, and when Cassandra bypasses it entirely — even when your WHERE clause references a SAI-indexed column.