Re: POC: Cleaning up orphaned files using undo logs

On Wed, May 22, 2019 at 7:17 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > +/* Extract xid from a value comprised of epoch and xid */
> > > +#define GetXidFromEpochXid(epochxid) \
> > > + ((uint32) (epochxid) & 0XFFFFFFFF)
> > > +
> > > +/* Extract epoch from a value comprised of epoch and xid */
> > > +#define GetEpochFromEpochXid(epochxid) \
> > > + ((uint32) ((epochxid) >> 32))
> > > +
> >
> > Why do these exist?
> >
>
> We don't need the second one (GetEpochFromEpochXid), but the first one
> is required. Basically, the oldestXidHavingUndo computation does
> consider oldestXmin (which is still a TransactionId) as we can't
> retain undo which is 2^31 transactions old due to other limitations
> like clog/snapshots still has a limit of 4-byte transaction ids.
> Slightly unrelated, but we do want to improve the undo retention in a
> subsequent version such that we won't allow pending undo for
> transaction whose age is more than 2^31.

The point is that we now have EpochFromFullTransactionId and
XidFromFullTransactionId. You shouldn't be inventing your own version
of that infrastructure. Use FullTransactionId, not a uint64, and then
use the functions for dealing with full transaction IDs from
transam.h.

--
Robert Haas
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