For a long time hash indexed were not crash safe, and couldn't be used on replication slave, because they skipped WAL. Now, thanks to these two commits, it has changed:
On 14th of March 2017, Robert Haas committed patch:
hash: Add write-ahead logging support. The warning about hash indexes not being write-ahead logged and their use being discouraged has been removed. "snapshot too old" is now supported for tables with hash indexes. Most importantly, barring bugs, hash indexes will now be crash-safe and usable on standbys. This commit doesn't yet add WAL consistency checking for hash indexes, as we now have for other index types; a separate patch has been submitted to cure that lack. Amit Kapila, reviewed and slightly modified by me. The larger patch series of which this is a part has been reviewed and tested by Álvaro Herrera, Ashutosh Sharma, Mark Kirkwood, Jeff Janes, and Jesper Pedersen. Discussion: http://postgr.es/m/CAA4eK1JOBX=YU33631Qh-XivYXtPSALh514+jR8XeD7v+K3r_Q@mail.gmail.com
and then, ~ 13 hours later, Robert committed also:
hash: Support WAL consistency checking. Kuntal Ghosh, reviewed by Amit Kapila and Ashutosh Sharma, with a few tweaks by me. Discussion: http://postgr.es/m/CAGz5QCJLERUn_zoO0eDv6_Y_d0o4tNTMPeR7ivTLBg4rUrJdwg@mail.gmail.com
Continue reading Waiting for PostgreSQL 10 – hash indexing vs. WAL