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

Michael Paquier wrote:
> On Fri, Oct 6, 2017 at 1:24 AM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> +   /*
> +    * If the xmax of the old tuple is identical to the xmin of the new one,
> +    * it's a match.
> +    */
> +   if (xmax == xmin)
> +       return true;
> I would use TransactionIdEquals() here, to remember once you switch
> that to a macro.

I've had second thoughts about the macro thing -- for now I'm keeping it
a function, actually.

> +/*
> + * Given a tuple, verify whether the given Xmax matches the tuple's Xmin,
> + * taking into account that the Xmin might have been frozen.
> + */
> [...]
> +   /*
> +    * We actually don't know if there's a match, but if the previous tuple
> +    * was frozen, we cannot really rely on a perfect match.
> +    */

I don't know what you had in mind here, but I tweaked the 9.3 version so
that it now looks like this:

/** HeapTupleUpdateXmaxMatchesXmin - verify update chain xmax/xmin lineage** Given the new version of a tuple after
someupdate, verify whether the* given Xmax (corresponding to the previous version) matches the tuple's* Xmin, taking
intoaccount that the Xmin might have been frozen after the* update.*/
 
bool
HeapTupleUpdateXmaxMatchesXmin(TransactionId xmax, HeapTupleHeader htup)
{TransactionId    xmin = HeapTupleHeaderGetXmin(htup);
/* * If the xmax of the old tuple is identical to the xmin of the new one, * it's a match. */if
(TransactionIdEquals(xmax,xmin))    return true;
 
/* * When a tuple is frozen, the original Xmin is lost, but we know it's a * committed transaction.  So unless the Xmax
isInvalidXid, we don't * know for certain that there is a match, but there may be one; and we * must return true so
thata HOT chain that is half-frozen can be walked * correctly. */if (TransactionIdEquals(xmin, FrozenTransactionId) &&
 TransactionIdIsValid(xmax))    return true;
 
return false;
}


-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers