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• Row ID - A monotonic increasing integer associated with every row in a sstable. It’s stored in an index structure instead of key token or key offset, because it compresses better.
• Postings/posting-list - Sorted row ids that match a given indexed value. 
• Token file - An index of Row ID -> partition key token for every row in the sstable.
• Offset file - An index of Row ID -> partition key offset on the data/primary-index file for every row in the sstable.
• Segment - A smallest unit of on-disk indexing structure that is flushed during compaction to reduce memory pressure. Multiple segments of an index are written to the same physical file.

Index Format Version 1

SAI is optimised for storage. Tokens and offsets are stored once per SSTable.  Column indexes access the token and offset files using a row ID. Offsets are compressed using Frame of Reference (FoR) encoding while tokens are not because tokens consume the full 8 bytes and therefore cannot be compressed.

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