NOTE
Index Data Structures
Why indexes trade extra storage/write work for faster lookup, range, search, or aggregation paths.
This is a historical learning note and may contain outdated or incomplete understanding.
An index is an auxiliary data structure that accelerates a query/access pattern by storing a searchable projection of the underlying data.
Examples include hash indexes for equality, B+ trees for equality/range/order, inverted indexes for term search, spatial indexes, and sparse/block indexes.
Indexes are not free: they consume space, add write/maintenance cost, and can amplify compaction/cache pressure. Design from the actual query patterns and consistency/update requirements.