Hash Index
A hash index is a data structure used in databases to enable fast lookup of records based on a key value, typically implemented using a hash table. It maps key values to specific storage locations (like disk pages or memory addresses) by applying a hash function to compute an index. This allows for constant-time O(1) average-case performance for exact-match queries, making it highly efficient for equality searches.
Developers should use hash indexes when they need to optimize for exact-match queries, such as in primary key lookups or unique constraints, where speed is critical and range queries are not required. They are particularly useful in in-memory databases, caching systems, or scenarios with high-frequency point queries, like session management or user authentication. However, they are not suitable for range scans or sorting operations, as the hash function does not preserve order.