In which cases is it appropriate to use a hash-based index in PostgreSQL?

In PostgreSQL, hash-based indexes are appropriate when you need to perform equality operations (=) very quickly on a specific column. Such indexes are efficient for exact match searches, for example, when searching by a unique identifier or key.

However, consider the following:

• Hash indexes do not support range queries (<, >, BETWEEN).
• In newer versions of PostgreSQL (starting from 10), hash indexes have become more reliable and support WAL (transaction log).
• For most cases, it is better to use a B-tree index, as it is more versatile.

Example of using a hash index:

CREATE INDEX idx_hash_userid ON users USING hash(user_id);

This is ideal if you frequently perform queries like:

SELECT * FROM users WHERE user_id = 12345;

If you need range queries or sorting, it is better to use a B-tree.