NOTE
Sparse Index
Indexing selected block/range boundary keys to reduce index size while requiring local scans within the selected region.
This is a historical learning note and may contain outdated or incomplete understanding.
A sparse index stores index entries for only some records/blocks rather than every record. Searching first locates the nearest indexed range/block, then scans or uses another structure within it.
Sparse indexes use less index memory/storage but require ordered data/range structure and additional local work compared with a dense per-record index.
They are common as one layer in storage-engine designs rather than as a universal standalone index.