Indexing Strategies

An index is a secondary data structure that lets the database find rows without scanning the whole table, trading write cost and storage for read speed. The choices that matter are index type (B-tree for ranges and sorting, hash for equality, covering for index-only scans), composite-index column order, and selectivity (an index on a low-cardinality column is often useless). Applied-AI interviews probe indexing because it is the first lever for a slow read, and the candidates who understand why the planner sometimes ignores an index are the ones who have actually tuned a database.