Regular 2i
Local index per node. Query hits all nodes. Fine for low-cardinality columns queried within known partition key.
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SASI
Advanced index supporting range + prefix. More flexible but higher write cost + still scatter-gather without partition key.
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SAI (Storage-Attached, C* 5.0)
New in Cassandra 5.0. Compact + fast. Better performance than SASI. Preferred going forward.