Re: found xmin from before relfrozenxid on pg_catalog.pg_authid

On 2018-May-24, Andres Freund wrote:

> On 2018-05-24 13:08:53 -0400, Alvaro Herrera wrote:
> > Hmm .. surely

> >      xid = HeapTupleHeaderGetXmin(tuple);
> >      xmin_frozen = ((xid == FrozenTransactionId) ||
> >                     HeapTupleHeaderXminFrozen(tuple));
> > -    if (TransactionIdIsNormal(xid))
> > +    if (!xmin_frozen && TransactionIdIsNormal(xid))

> I don't think that's necesary - HeapTupleHeaderGetXmin() returns
> FrozenTransactionId if the tuple is frozen (note the
> HeapTupleHeaderXminFrozen() within).

Ah, yeah ... I probably thought about this when writing it and removed
it for that reason.

BTW I think the definition of HeapTupleHeaderXminFrozen is seriously
confusing, by failing to return true if the xmin is numerically
FrozenXid (which it'll be if the database was pg_upgraded).  I wonder
about this one in HeapTupleSatisfiesMVCC:

    else
    {
        /* xmin is committed, but maybe not according to our snapshot */
        if (!HeapTupleHeaderXminFrozen(tuple) &&
            XidInMVCCSnapshot(HeapTupleHeaderGetRawXmin(tuple), snapshot))
            return false;        /* treat as still in progress */
    }

I think this is not a bug only because XidInMVCCSnapshot does this

    /* Any xid < xmin is not in-progress */
    if (TransactionIdPrecedes(xid, snapshot->xmin))
        return false;

which makes it return false for FrozenXid, but seems more of an accident
than explicitly designed.

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