Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Thu, Sep 28, 2017 at 2:39 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>> FWIW, I am reminded a little bit of the MultiXact/recovery bug I
>> reported way back in February of 2014 [1], which also had a HOT
>> interaction that caused index scans to give wrong answers, despite
>> more or less structurally sound indexes.
>>
>> [1] https://www.postgresql.org/message-id/CAM3SWZTMQiCi5PV5OWHb+bYkUcnCk=O67w0cSswPvV7XfUcU5g@mail.gmail.com
>
> Thanks for the reference. I didn't remember this problem and it's not
> (wasn't) in my list of things to look into. Perhaps these are both the
> same bug.

I was reminded of that old bug because initially, at the time, it
looked very much like a corrupt index: sequential scans were fine, but
index scans gave wrong answers. This is what I saw today.

In the end, commit 6bfa88a fixed that old recovery bug by making sure
the recovery routine heap_xlog_lock() did the right thing. In both
cases (Feb 2014 and today), the index wasn't really corrupt -- it just
pointed to the root of a HOT chain when it should point to some child
tuple (or maybe a successor HOT chain).

--
Peter Geoghegan