On 03/31/2017 11:19 AM, Magnus Hagander wrote:
> On Fri, Mar 31, 2017 at 8:17 PM, Robert Haas <robertmh...@gmail.com
> <mailto:robertmh...@gmail.com>> wrote:
>     Starting a new thread about this to get more visibility.
>     Despite the extensive work that has been done on hash indexes this
>     release, we have thus far not made any change to the on-disk format
>     that is not nominally backward-compatible.  Commit
>     293e24e507838733aba4748b514536af2d39d7f2 did make a change for new
>     hash indexes, but included backward-compatibility code so that old
>     indexes would continue to work.  However, I'd like to also commit
>     Mithun Cy's patch to expand hash indexes more gradually -- latest
>     version in
> http://postgr.es/m/cad__oujd-ibxm91zcqziayftwqjxnfqgmv361v9zke83s6i...@mail.gmail.com
> <http://postgr.es/m/cad__oujd-ibxm91zcqziayftwqjxnfqgmv361v9zke83s6i...@mail.gmail.com>
>     -- and that's not backward-compatible.
>     It would be possible to write code to convert the old metapage format
>     to the new metapage format introduced by that patch, and it wouldn't
>     be very hard, but I think it would be better to NOT do that, and
>     instead force everybody upgrading to v10 to rebuild all of their hash
>     indexes.   If we don't do that, then we'll never know whether
>     instances of hash index corruption reported against v10 or higher are
>     caused by defects in the new code, because there's always the chance
>     that the hash index could have been built on a pre-v10 version, got
>     corrupted because of the lack of WAL-logging, and then been brought up
>     to v10+ via pg_upgrade.  Forcing a reindex in v10 kills three birds
>     with one stone:
>     - No old, not logged, possibly corrupt hash indexes floating around
>     after an upgrade to v10.
>     - Can remove the backward-compatibility code added by
>     293e24e507838733aba4748b514536af2d39d7f2 instead of keeping it around
>     forever.
>     - No need to worry about doing an in-place upgrade of the metapage for
>     the above-mentioned patch.
>     Thoughts?
> Given the state of hash indexes in <= 9.6, I think this is a reasonable
> tradeoff. Most people won't be using them at all today. Those that do
> will have to "pay" with a REINDEX on upgrade. I think the benefits
> definitely outweigh the cost.
> So +1 for doing it. 


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