Hash indexes

Hash indexes have been around for many years. The idea is to hash the input value and store it for later lookups. Having hash indexes actually makes sense. However, before PostgreSQL 10.0, it was not advisable to use hash indexes because PostgreSQL had no WAL support for them. In PostgreSQL 10.0, this has changed. Hash indexes are now fully logged and are therefore ready for replication and considered to be 100% crash-safe.

Hash indexes are generally a bit larger than B-tree indexes. Suppose you want to index 4 million integer values. A B-tree will need around 90 MB of storage to do this. A hash index will need around 125 MB on disk. The assumption that's made by many people is that a hash is super small on disk and therefore, in many cases, the assumption can be wrong as well.