Document not using database hash indexes

doc/development/README.md

@@ -59,6 +59,7 @@
 - [Iterating Tables In Batches](iterating_tables_in_batches.md)
 - [Ordering Table Columns](ordering_table_columns.md)
 - [Verifying Database Capabilities](verifying_database_capabilities.md)
+- [Hash Indexes](hash_indexes.md)
 
 ## i18n
 

doc/development/hash_indexes.md

+# 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.