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

On Fri, Oct 6, 2017 at 11:34 AM, Peter Geoghegan <pg@bowt.ie> wrote:
>> I don't know if it's really the freeze map at fault or something else.
>
> Ideally, it would be possible to effectively disable the new freeze
> map stuff in a minimal way, for testing purposes. Perhaps the authors
> of that patch, CC'd, can suggest a way to do that.

Actually, the simplest thing might be to just use pg_visibility's
pg_check_frozen() to check that the visibility/freeze map accurately
summarizes the all-frozen status of tuples in the heap. If that
doesn't indicate that there is corruption, we can be fairly confident
that the problem is elsewhere. The metadata in the visibility/freeze
map should be accurate when a bit is set to indicate that an entire
heap page is all-frozen (or, separately, all-visible). We can hardly
expect it to have better information that the authoritative source of
truth, the heap itself.

The more I think about it, the more I tend to doubt that the remaining
problems are with the freeze map. If the freeze map was wrong, and
incorrectly said that a page was all-frozen, then surely the outward
symptoms would take a long time to show up, as they always do when we
accidentally fail to freeze a tuple before a relfrozenxid cutoff. ISTM
that that's the only meaningful way that the freeze map can be wrong
-- it only promises to be accurate when it says that no further
freezing is needed for a page/bit.

-- 
Peter Geoghegan


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