Since I just committed the last pending patch to improve hash indexes to PostgreSQL 11, and since most of the improvements to hash indexes were committed to PostgreSQL 10 which is expected to be released next week, it seems like a good time for a brief review of all the work that has been done over the last 18 months or so. Prior to version 10, hash indexes didn't perform well under concurrency, lacked write-ahead logging and thus were not safe in the face either of crashes or of replication, and were in various other ways second-class citizens. In PostgreSQL 10, this is largely fixed.
