Re: new heapcheck contrib module

On Mon, Jul 20, 2020 at 5:02 PM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
> I've made the options 'all-visible', 'all-frozen', and 'none'. It defaults to 'none'.

That looks nice.

> > I guess that
> > could still be expensive if there's a lot of them, but needing
> > ShareUpdateExclusiveLock rather than only AccessShareLock is a little
> > unfortunate.
>
> I welcome strategies that would allow for taking a lesser lock.

I guess I'm not seeing why you need any particular strategy here. Say
that at the beginning you note the starting relfrozenxid of the table
-- I think I would lean toward just ignoring datfrozenxid and the
cluster-wide value completely. You also note the current value of the
transaction ID counter. Those are the two ends of the acceptable
range.

Let's first consider the oldest acceptable XID, bounded by
relfrozenxid. If you see a value that is older than the relfrozenxid
value that you noted at the start, it is definitely invalid. If you
see a newer value, it could still be older than the table's current
relfrozenxid, but that doesn't seem very worrisome. If the user
vacuumed the table while they were running this tool, they can always
run the tool again afterward if they wish. Forcing the vacuum to wait
by taking ShareUpdateExclusiveLock doesn't actually solve anything
anyway: you STILL won't notice any problems the vacuum introduces, and
in fact you are now GUARANTEED not to notice them, plus now the vacuum
happens later.

Now let's consider the newest acceptable XID, bounded by the value of
the transaction ID counter. Any time you see a newer XID than the last
value of the transaction ID counter that you observed, you go observe
it again. If the value from the table still looks invalid, then you
complain about it. Either way, you remember the new observation and
check future tuples against that value. I think the patch is already
doing this anyway; if it weren't, you'd need an even stronger lock,
one sufficient to prevent any insert/update/delete activity on the
table altogether.

Maybe I'm just being dense here -- exactly what problem are you worried about?

--
Robert Haas
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