Re: [COMMITTERS] pgsql: Fix traversal of half-frozen update chains

Peter Geoghegan wrote:
> On Fri, Oct 6, 2017 at 8:29 AM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:

> > /*
> > * When a tuple is frozen, the original Xmin is lost, but we know it's a
> > * committed transaction. So unless the Xmax is InvalidXid, we don't know
> > * for certain that there is a match, but there may be one; and we must
> > * return true so that a HOT chain that is half-frozen can be walked
> > * correctly.
> > *
> > * We no longer freeze tuples this way, but we must keep this in order to
> > * interpret pre-pg_upgrade pages correctly.
> > */
> > if (TransactionIdEquals(xmin, FrozenTransactionId) &&
> > TransactionIdIsValid(xmax))
> > return true;
> >
> > return false;
> > }
>
> Wouldn't this last "if" test, to cover the pg_upgrade case, be better
> targeted by comparing *raw* xmin to FrozenTransactionId? You're using
> the potentially distinct xmin value returned by
> HeapTupleHeaderGetXmin() for the test here. I think we should be
> directly targeting tuples frozen on or before 9.4 (prior to
> pg_upgrade) instead.

Yes, agreed, I should change that. Thanks for continuing to think about
this.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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