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Hash Indexes
Both PostgreSQL and MySQL support hash indexes besides the regular btree indexes. Hash indexes however are to be avoided at all costs. While they may sometimes provide better performance the cost of rehashing can be very high. More importantly: at least until PostgreSQL 10.0 hash indexes are not WAL-logged, meaning they are not replicated to any replicas. From the PostgreSQL documentation:
Hash index operations are not presently WAL-logged, so hash indexes might need to be rebuilt with REINDEX after a database crash if there were unwritten changes. Also, changes to hash indexes are not replicated over streaming or file-based replication after the initial base backup, so they give wrong answers to queries that subsequently use them. For these reasons, hash index use is presently discouraged.
RuboCop is configured to register an offence when it detects the use of a hash index.
Instead of using hash indexes you should use regular btree indexes.