Re: Hash Indexes

On Thu, Sep 15, 2016 at 7:13 AM, Robert Haas <robertmhaas@gmail.com> wrote:

> On Thu, Sep 15, 2016 at 1:41 AM, Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> > I think it is possible without breaking pg_upgrade, if we match all
> > items of a page at once (and save them as local copy), rather than
> > matching item-by-item as we do now.  We are already doing similar for
> > btree, refer explanation of BTScanPosItem and BTScanPosData in
> > nbtree.h.
>
> If ever we want to sort hash buckets by TID, it would be best to do
> that in v10 since we're presumably going to be recommending a REINDEX
> anyway.


We are?  I thought we were trying to preserve on-disk compatibility so that
we didn't have to rebuild the indexes.

Is the concern that lack of WAL logging has generated some subtle
unrecognized on disk corruption?

If I were using hash indexes on a production system and I experienced a
crash, I would surely reindex immediately after the crash, not wait until
the next pg_upgrade.



> But is that a good thing to do?  That's a little harder to
> say.
>

How could we go about deciding that?  Do you think anything short of coding
it up and seeing how it works would suffice?  I agree that if we want to do
it, v10 is the time.  But we have about 6 months yet on that.

Cheers,

Jeff