Re: 32/64-bit transaction IDs?

On Saturday March 22 2003 9:15, Ed L. wrote:
> On Saturday March 22 2003 8:44, Tom Lane wrote:
> > "Ed L." <pgsql@bluepolka.net> writes:
> > >     create table pg_xlog (
> > >         xid        int4 unique not null,
> > >         recno    int8 unique not null,
> > >     );
> > >     -- recno = (XLogRecPtr.xlogid) << 32) + XLogRecPtr.xrecoff
> > >
> > > This would map transaction IDs to WAL log record numbers.  It seems
> > > straight-forward to get the right data into this table.
> >
> > ... except that once a transaction has committed, it can't go around
> > making more database entries.  You can only modify the DB within a
> > transaction.
>
...
>
> 2.  In xact.c, RecordTransactionCommit() inserts the (xid,recno) pair
> into pg_xlog just after XLogInsert() for the commit.  It doesn't matter
> if it's within reach of the user's transaction-constrained trigger code
> or not because it's done by the system.

Am I correct in assuming the system can modify system tables outside a
transaction?