Re: Improve search for missing parent downlinks in amcheck

On Tue, Apr 16, 2019 at 10:04 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Apr 16, 2019 at 12:00 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > Can you be more specific? What was the cause of the corruption? I'm
> > always very interested in hearing about cases that amcheck could have
> > detected, but didn't.
>
> FWIW, v4 indexes in Postgres 12 will support the new "rootdescend"
> verification option, which isn't lossy, and would certainly have
> detected your customer issue in practice. Admittedly the new check is
> quite expensive, even compared to the other bt_index_parent_check()
> checks, but it is nice that we now have a verification option that is
> *extremely* thorough, and uses _bt_search() directly.

"rootdescend" is cool type of check.  Thank you for noticing, I wasn't
aware of it.
But can it detect the missing downlink in following situation?

        A
     /     \
  B <-> C <-> D

Here A has downlinks to B and D, which downlink to C is missing,
while B, C and D are correctly connected with leftlinks and rightlinks.
I can see "rootdescend" calls _bt_search(), which would just step
right from C to D as if it was concurrent split.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company